6
Gem Identification Magnification
Table of Contents
Subject
Page
Loupes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 Loupe Lighting Techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 Gemological Microscopes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 Focusing the Microscope . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 Microscope Lighting Techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 Darkfield Illumination . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 Brightfield Illumination . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20 Reflected Lighting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22 Diffused Lighting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22 Polarized Lighting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 Examining a Stone . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24 Identifying Clarity Characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28 Surface Characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29 Inclusions in Natural Gems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32 Inclusions in Synthetics and Imitations . . . . . . . . . . . . . . . . . . . . . . 35 Identifying Assembled Stones . . . . . . . . . . . . . . . . . . . . . . . . . . 37 Key Concepts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39 Key Terms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40
©
The Gemological Institute of America Carlsbad, CA 92008 ©2003 The Gemological Institute of America All rights reserved: Protected under the Berne Convention. No part of this work may be copied, reproduced, transferred, or transmitted in any form or by any means whatsoever without the express written permission of GIA. Printed in the United States.
Cover photos: All by Terri Weimer/GIA
Facing page: Magnification makes this included crystal visible. Its presence proves that the host sapphire is a natural gem.
Mike Havstad/GIA
MAGNIFICATION “I know it’s a ruby—I get the right RI readings, and the dichroscope confirms it’s doubly refractive,” said Mike. “But when I look at it under the microscope, I can’t tell whether it’s natural or synthetic.” “I know. It’s getting tougher to separate treated natural rubies from synthetic ones. But let me take a look,” said Joe. “There’s usually something— some mineral crystals, clouds, or needles—that proves it’s a natural gem.” Joe examined the ruby carefully under the microscope. “I can see a couple of melted mineral crystals close to the girdle, and that whitish cloud is probably the remains of some growth zoning.”
Alan Jobbins
A whitish, hexagonal area is a feature that identifies some heat-treated, natural Mong Hsu rubies.
“But what about that area under the table—doesn’t that look like flux to you?” asked Mike. “It looks more like the remnants of heat treatment,” replied Joe. “When you’ve seen as many treated natural rubies as I have, you’ll recognize them quickly. Looking at a lot of them is the best way to become familiar with their characteristics.”
©2003 GIA. All rights reserved.
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GEM IDENTIFICATION
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Duncan Pay
John Koivula/GIA
These inclusions look similar, but one is a flux inclusion in a synthetic ruby (above) and the other is a borax inclusion in the partially healed fracture of a heat-treated natural ruby (left).
Dietmar Schwarz
As gem crystals grow, they often trap mineral inclusions from their environment. The included minerals help identify a gem as natural. Magnification at 50X reveals stalklike green amphibole crystals in this natural emerald.
Inclusion—A characteristic enclosed within a gemstone or reaching its surface from the interior. Blemish—Characteristic or irregularity confined to the surface of a polished gemstone.
2
Like Mike’s ruby, many gems carry signs of their nature in the form of characteristics called inclusions, which are enclosed within a gem or reach its surface from the interior. Some types of inclusions—like mineral crystals—are remnants of the rocks that natural gems grow in. Others— like curved color banding and platinum platelets—are found in synthetic gems and hint at the processes used by manufacturers to grow them. Blemishes can help with identification, too. Blemishes are characteristics like scratches and abrasions on a polished gem’s surface. And they can often indicate a gem’s hardness. For example, a fairly soft gem like
MAGNIFICATION
Magnification is a valuable tool for detecting treatments and for separating natural gems from their synthetic counterparts.
Alan Jobbins
You’ll usually see abraded facet junctions on gems that don’t rate very high on the Mohs hardness scale. This demantoid garnet shows abrasions on its crown and pavilion facet junctions, along with a prominent horsetail inclusion.
Both by Nicholas DelRe/GIA
This pendant (right) contains diamonds, natural rubies, and synthetic rubies. Magnification reveals gas bubbles in the stone at bottom right (above), indicating it’s a synthetic ruby.
demantoid garnet (Mohs 6.5) often has abraded facet edges, while a hard gem like corundum (Mohs 9) usually doesn’t. Magnification can help you determine if a gem is treated or if it contains internal fractures, vulnerable cleavages, or other structural defects. It’s also an important tool for separating natural gems from synthetics. This is a vital separation because there’s such a large value difference between many synthetic gems and their natural counterparts of equivalent quality. For example, it’s easy to separate emerald from other green gems like chrome tourmaline, chrome diopside, green sapphire, and peridot using 3
GEM IDENTIFICATION
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You can keep up to date with the constantly changing gem world by reading trade publications like Gems & Gemology.
Terri Weimer/GIA
A gemological microscope is more versatile and provides greater magnification than a loupe, but a loupe is much more portable. GIA Gem Instruments carries a variety of loupes and microscopes.
Practical experience and up-to-date knowledge are the keys to using magnification successfully in gem identification.
the refractometer because each gem has a very different refractive index (RI). It’s much more challenging to tell if an emerald is natural or synthetic. That’s because the physical and optical properties of many natural and synthetic stones—including emerald—overlap. Magnification can be a very powerful tool, and the more you practice using it, the more skilled you’ll become at recognizing the features that help you make a final determination. But it’s also important to keep up with the latest industry information by reading gemological business and scientific journals. Gemologists use two types of magnifiers: loupes and microscopes. Loupes are small, easy-to-carry magnifiers that come in a variety of forms. Microscopes are much more sophisticated and capable of far greater magnification, but they’re much less portable.
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MAGNIFICATION
Reporters Press Agency/eStock Photo
The hand loupe’s lens cover serves as a handle when you examine a stone. The cover protects the lens when the loupe isn’t in use.
LOUPES ■ What’s the most popular loupe for gem identification? ■ How do you light a gem to examine its surface? ■ How do you light a gem to examine its interior?
A loupe’s portability and affordability make it a versatile tool for gem identification. You learned how to use a loupe in Assignment 2. By practicing the technique outlined there, you can master the loupe and maximize its effectiveness in the gem identification process. Some jewelers use an eye loupe that attaches to eyeglasses or fits in an eye socket like a monocle, leaving both hands free to examine a stone or to work on a jewelry piece. But today, most gem professionals use hand loupes. A hand loupe has a cover that doubles as a handle. 5
GEM IDENTIFICATION
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A fully corrected 10X triplet loupe is an essential gemological tool.
Terri Weimer/GIA
These are three of the 10X loupes available from GIA Gem Instruments. Each one has its advantages. The smaller hand loupe is compact, while the larger one gives a greater field of view. The darkfield loupe at the top provides darkfield illumination, which makes it easier to identify many inclusions.
Spherical aberration—Blurring around the edges that occurs when a lens can’t get an entire image in focus at the same time. Chromatic aberration—Color distortion caused by the inability of a lens to bring the various colored wavelengths of light into focus at the same distance. Fully corrected triplet loupe—A loupe that contains a three-part lens that magnifies and corrects for spherical and chromatic aberration.
Loupes come in powers from 2-power (2X) to 30-power (30X). Under 2X magnification, the diameter of the image is 2 times greater than the diameter of the object you’re magnifying; under 30X, the image’s diameter is 30 times the object’s diameter. The most widely used loupe in the jewelry industry is the 10X loupe. But not just any 10X loupe will do. It must be a good-quality instrument to be useful for grading and testing gems. If you look at a gem through a low-quality loupe, you’ll notice that the facet edges are in focus at the center of the lens, but appear blurred around the edges. This is called spherical aberration, and it occurs because the lens can’t keep the entire image in focus at the same time. Another form of distortion occurs when a lens can’t focus all the colored wavelengths of white light at the same point. This effect is called chromatic aberration, and it causes fringes of color around lines such as facet edges. If you look at a diamond through such a lens, chromatic aberration might mislead you about its color. Good-quality loupes cure these distracting optical effects by using three lenses joined together into one unit. One lens acts as a magnifier, another corrects for spherical aberration, and the third corrects for chromatic aberration. This kind of a loupe is called a fully corrected triplet loupe. You’ll need a 10X triplet loupe to examine, identify, and grade gems. Because they’re convenient, portable, and inexpensive, loupes are perfect for buying trips. But their relatively low magnifying power can also be a challenge. Standard 10X magnification is fine for most grading tasks, when you have to judge the effects of inclusions on appearance. But at that magnification level, it’s often difficult to identify the inclusions that distinguish natural gems from synthetics.
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MAGNIFICATION
Focal distance—The distance from the surface of a lens to a point that’s in sharp focus.
Terri Weimer/GIA
An uncorrected loupe shows both spherical and chromatic aberration.
Peter Johnston/GIA
A fully corrected triplet loupe uses a three-part lens to correct both spherical and chromatic aberration.
Working distance—the distance from the lens to the surface of the object—also affects a loupe’s usefulness. It’s determined by the loupe’s focal distance, which is the distance from the surface of the lens to a point that’s in sharp focus. The higher the magnification, the shorter the focal distance and working distance. A 10X loupe focuses when an object is one inch away. A 20X loupe doubles the magnification, but cuts the focal distance in half, which means it focuses when the object is half an inch away. This also cuts the working distance in half, leaving less of a margin before the stone or its characteristics are out of focus. At 30X, the working distance is even smaller. 7
GEM IDENTIFICATION
Depth of field—The distance that’s clear and sharp in front of and behind the point you focus on.
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DEPTH OF FIELD VIEWER
working distance
focal distance
depth of field LOW MAGNIFICATION
HIGH MAGNIFICATION
Peter Johnston/GIA
When you use magnification to examine a gem, you have to consider the working distance from the lens to the gem, the focal distance from the lens to the characteristic you’re examining, and depth of field, which is the area in front of and behind the object you’re examining. When you switch to higher magnification, you shorten the working distance, focal distance, and depth of field.
This makes loupes with magnifications higher than 10X more difficult to use effectively. The higher the loupe’s magnification, the closer you have to get to the gem, and the harder it is to focus on an individual feature within the stone. The shortened working distance at higher powers also leaves less room for lighting and makes it more difficult to light a stone effectively. Another disadvantage is that it creates a shallower depth of field. Depth of field is the distance that’s sharp and clear in front of and behind the point—such as a small inclusion—that you’re focusing on. With higher-power loupes, the depth of field is very small. To keep an object in focus, you need to keep both the loupe and the stone as still as possible. Another consequence of higher magnification is that the area of the gem that you can examine—the field of view—becomes smaller. Despite these limitations, the loupe can be an amazingly revealing instrument. 8
MAGNIFICATION
Reflected light is best for examining a stone’s surface. Darkfield illumination works best for examining the interiors of transparent stones.
Eric Welch/GIA
Darkfield lighting lets you examine the interior of a transparent stone. You can create it by taping a sheet of black paper to the back edge of a lampshade.
LOUPE LIGHTING TECHNIQUES
There are two basic types of lighting you can use to examine transparent gem materials with a loupe. One lets you examine the gem’s surface while the other lets you see internal inclusions more clearly.
Reflected lighting—Illumination of a gem’s surface by reflecting light from it.
The first is reflected lighting—strong, direct lighting reflected off a gem’s surface. Under reflected light, features like surface-reaching fractures, abrasions, and cavities stand out strongly against the gem’s polished surface.
Darkfield illumination—Lighting of a gemstone from the side against a black, non-reflective backround.
You can use almost any strong light source, such as a desk lamp, a high-intensity lamp, or a fiber-optic light source like a FiberLite. Hold the stone face-up with tweezers and position the light source and the stone so the light reflects off the stone’s surface. Tilt the gem until each facet in turn shows a bright, shiny, reflective surface. Examine the stone face-up first and then keep turning the stone until you’ve examined it from every side. The second technique—darkfield illumination—lets you see into a gem’s interior. With this form of illumination, inclusions within the gem stand out strongly against a dark background. 9
GEM IDENTIFICATION
6 You can use a desktop lamp and black paper to create darkfield illumination for a standard 10X loupe. Follow these steps: 1. Use tape strips to hang a sheet of black, non-reflective paper from the back edge of the lampshade. 2. Turn the room lights off. Turn the lamp on, and direct the light straight down. 3. Hold the stone face-up with tweezers. Position the stone so the girdle plane lines up approximately with the front edge of the lampshade. 4. Examine the stone’s interior against the black background, adjusting the stone’s position to find the best view. Keep the stone in the light and the loupe out of the light. This directs light in from the sides of the stone without creating too many distracting reflections off the crown. If you’re trying to distinguish diffusion treatment, a third type of lighting—diffused lighting—can be helpful. You can create diffused lighting by placing a sheet of translucent white material, such as a facial tissue, between the light source and the stone.
Eric Welch/GIA
You can create diffused lighting by taping a translucent white sheet to the front of a lampshade. It can help you detect the characteristic color zoning in diffusion-treated corundum and curved color banding in flame-fusion synthetic corundum.
Using a loupe, you can perform almost any basic magnification test. But for more powerful magnification needs, the gemological microscope is the instrument of choice.
GEMOLOGICAL MICROSCOPES ■ Why is a binocular microscope the best choice for gem
identification? ■ What’s the best way to focus a gem microscope? ■ What are the various types of lighting needed for
examining gems?
Pod—The housing for a microscope’s optical system, also called the head.
With its sophisticated optical system, sturdy construction, integrated lighting, and greater working distance, the gemological microscope can almost always help you identify treatments and make the vital separation between natural and synthetic gemstones. Most microscopes designed for grading or identifying gems are binocular, which means they have two sets of lenses. The binocular optical system has a great advantage over the monocular system, which has only one lens set. The binocular system produces a three-dimensional image with normal orientation. This makes manipulating the gem much easier. Most monocular systems produce images that are flat, upside down, and reversed. There are four basic parts to a typical binocular gemological microscope. The housing at the top of the microscope that contains the optical components is called the pod. You can move the pod up and down to change the instrument’s focus. You do this by turning the focus-control knob on the instrument’s arm.
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MAGNIFICATION
Stage—A microscope’s working platform.
GEMOLOGICAL MICROSCOPE
Light well—Housing for a microscope’s light source, located below the stage.
pod
zoom adjustment
oculars
objectives
Base—The support platform that contains a microscope’s electrical controls. Oculars—The eyepiece lenses on a gemological microscope.
overhead light
focus adjustment
Objectives—The lenses nearest the stone on a gemological microscope.
stage
light well
base overhead light switch
rheostat for light well
A binocular microscope—like this one from GIA Gem Instruments—is a superior tool for any gemological task that requires magnification.
Directly below the pod, there’s a working platform called the stage, where you place the gem for examination. Beneath that, there’s housing for a light source called the light well. Finally, there’s the base, which supports the microscope and contains the electrical controls. The pod contains a complex system of lenses and prisms. The eyepiece lenses are called the oculars, and the lenses nearest the stone are the objectives. Most microscopes have eyepieces that you can adjust for individual comfort. Some are equipped with plastic or rubber eyecups that help eliminate extraneous light, keep your eyes at the correct distance from the oculars, and make the microscope more comfortable to use. The eyecups are removable, and some gemologists, especially those who wear glasses, prefer to work without them.
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GEM IDENTIFICATION
6 You can determine the power of a microscope’s magnification by multiplying the power of the oculars by the power of the objectives. For example, 10X oculars and 2X objectives give 20X magnification. The typical range is 10X to 30X or 45X, but some models go up to 70X or more. With some gem microscopes, you change magnification powers by switching oculars. With others, you turn the objectives to one of several settings (1X, 2X, or 3X, for example). But the most versatile models let you zoom smoothly from one power to another. The zoom adjustment is usually on the side or top of the pod. It can be a single knob or a pair of knobs, with one on either side of the pod. A calibrated dial displays the magnification of the objectives.
Terri Weimer/GIA
The stoneholder attaches to the side of the microscope’s stage and holds the stone in place. The iris diaphragm at the top of the light well consists of a series of metal plates that slide together or apart to control the amount of light coming up through the light well. You operate it by moving the lever on the left. At the bottom of the light well, there’s a metal flap called the baffle. When it’s closed, it prevents light from shining directly through the stone. You operate it by turning the knob on the right.
Iris diaphragm—A device in the microscope’s stage that you can open or close to control the amount of light coming from the light well. Stoneholder—A device that attaches to a microscope’s stage to hold a gem steady. Baffle—A small metal plate that you can close to prevent the microscope’s light from shining directly through the stone from below.
Many models let you attach a doubler, which is a 2X lens that screws on under the objectives, doubling the power. You can accomplish most gem identification with 10X to 45X magnification. Very few identifications require more than 90X. While higher magnification decreases the depth and width of field in a microscope, its depth and width of field are much larger than a loupe’s to begin with, so there are fewer practical problems. As with a loupe, higher magnification makes it more difficult to light the stone properly, but the lighting systems built into many gemological microscopes provide effective illumination at higher powers. The microscope’s stage has an opening that allows light to pass through from the light well below. Most gem microscopes have an iris diaphragm at the top of the stage that you can open or close to control the amount of light that comes up from the light well. The stage might also have a number of sockets where you can attach a stoneholder, which has spring-loaded jaws designed to hold a gem. Because it attaches to the stage, it holds the gem firmly in place and leaves both hands free to operate the microscope or to record what you see. If you use tweezers rather than a stoneholder, rest them against the edge of the light well on the microscope’s stage to hold your gemstone steady. There’s often an overhead fluorescent light source—a removable source of daylight-equivalent light—mounted at the front of the stage. The light well consists of a frosted glass or plastic cylinder inside a reflective metal bowl. The microscope’s light source is positioned at the base of the bowl, immediately below the cylinder. Above that, at the base of the cylinder, there’s a baffle—a small metal flap that can be opened or closed. When it’s open, light comes directly through the opening to light the stone from below. When it’s closed, the light is forced to come up from the sides of the light well rather than through the opening. As you’ll see, this is essential for darkfield illumination. The microscope’s base contains most of its electronics. On the back of the base is a small knob called a rheostat that turns the internal light bulb on or off and also controls the light’s intensity.
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MAGNIFICATION
All by Eric Welch/GIA
To set up a microscope’s focus, begin by turning the well light on (top left). Next, make sure the baffle is closed and the iris diaphragm has only a small opening (bottom left). Then, using 10X magnification and both eyes, turn the focus-control knob to focus on the edges of the small opening in the iris diaphragm (right).
FOCUSING THE MICROSCOPE
It’s important to focus your microscope before you begin each work session. You should also refocus it periodically during long work sessions. Follow these steps: 1. Turn the well light on. Make sure the baffle is closed and the iris diaphragm is shut down to a small opening. At 10X magnification and using both eyes, focus on the edges of the iris diaphragm’s opening, which is near the center of the field of view. Most gem microscopes have one ocular (usually the left) that you can focus without the focus-control knob. Remove that ocular and look through the right ocular, keeping both eyes open. 13
GEM IDENTIFICATION
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All by Eric Welch/GIA
Continue by removing the left ocular and looking through the right ocular. Keep both eyes open. Turn the focus-control knob until the edges of the opening in the iris diaphragm are in sharp focus.
Next, replace the left ocular and remove the right ocular. With both eyes open, look through the left ocular. Focus on the edges of the iris diaphragm opening by turning the ring at the bottom of the left ocular.
2. Turn the focus-control knob until the iris diaphragm’s opening is in focus in your right eye. Release the focus-control knob. Don’t touch it again until after you focus the left ocular. 3. Replace the left ocular. Remove the right ocular and look through the left ocular, keeping both eyes open. Bring the iris diaphragm’s opening into focus by turning the ring at the bottom of the left ocular.
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MAGNIFICATION
Continue by replacing the right ocular and adjusting the distance between the oculars to suit your eyes.
Finally, look through both oculars at the same time to confirm sharp focus on the edges of the iris diaphragm opening.
4. Replace the right ocular. Adjust the distance between the oculars for your comfort. Confirm the focus by looking at the opening in the iris diaphragm through both oculars at the same time. You should see a single image, and it should look sharp and three dimensional. If it doesn’t, repeat the focusing process.
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GEM IDENTIFICATION
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Eric Welch/GIA
A GIA Gem Instruments FiberLite, like other fiber-optic sources of condensed light, is especially good for horizontal, oblique, and pinpoint lighting. Fiber-optic light is valuable for identifying treatments and for separating synthetic from natural gems.
MICROSCOPE LIGHTING TECHNIQUES
A microscope offers many more lighting options than a loupe, and different types of lighting work best for seeing different gem features. The most useful techniques are darkfield, brightfield, diffused, reflected, and polarized lighting. An overhead light source makes it easier to see surface characteristics. Some microscopes come with a small fluorescent light or let you attach one to the stage for this purpose. You can also use a desk lamp as you would with a loupe. A fiber-optic system like the FiberLite is a versatile supplementary light source that’s vital for some separation processes. Varying the lighting can have a dramatic effect on the visibility and appearance of characteristics, and what you can determine by examining them. Surface characteristics visible in reflected light are invisible in darkfield light. And internal characteristics visible in darkfield light are invisible in reflected light. 16
MAGNIFICATION
DARKFIELD ILLUMINATION
stoneholder microscope stage
baffle closed to create dark background for stone
With the baffle closed, no light enters the stone from below. Light from the sides makes inclusions stand out dramatically against a dark background.
light source
Peter Johnston/GIA
Most gemological microscopes are designed with a baffle to provide the option of darkfield lighting.
DARKFIELD ILLUMINATION
Most gem microscopes have the built-in ability to provide darkfield illumination for examining inclusions. You just have to turn on the microscope’s internal light source and close the baffle in the light well so no light can enter the stone from directly below. Light enters the stone from the sides and a little behind, making inclusions stand out brightly against a dark background.
Relief—Contrast between an inclusion and its host gem. Included crystal—A mineral crystal trapped within a gem as it grows.
The degree to which a characteristic stands out against the surrounding gemstone is called its relief. For example, included crystals are minerals trapped within a gem as it grows. The brassy, metallic surfaces of pyrite included crystals stand out readily in pale emerald, so they’re described as having high relief. An included crystal’s relief depends on its RI and often its color, especially compared to the color of the host gem. A cluster of moderately sized, colorless calcite inclusions in a blue sapphire might be much harder 17
GEM IDENTIFICATION
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Alan Jobbins
Karl Schmetzer
Low-relief inclusions like this spinel crystal in ruby (above) are not as easy to see as high-relief inclusions like the black chromite crystal in peridot (left).
Liquid inclusion—Pocket in a gem that’s filled with fluids and, sometimes, gas bubbles and crystals.
John Koivula/GIA
Low-relief inclusions like this one, which contains a liquid, a gas, and a tiny crystal, are common in some emeralds from Colombia.
to see than a few black chromite crystals scattered around the interior of a pale green peridot. Most included crystals are relatively easy to see under darkfield illumination. Other characteristics, like liquid inclusions—pockets in gems filled with fluids and sometimes other materials—might require different lighting techniques because they tend to blend into the host 18
MAGNIFICATION
John Koivula/GIA
Horizontal lighting reveals minute flux particles in a Kashan synthetic ruby.
Both by Eric Welch/GIA
By using a fiber-optic light source to illuminate a stone horizontally and examining the gem with the well light both on (left) and off (right), you can see inclusions that might otherwise go undetected.
Eric Welch/GIA
Oblique lighting places the illumination from a fiber-optic light at an angle between horizontal and overhead.
gem’s background if you use darkfield. Horizontal lighting is a pinpoint darkfield technique, where you direct a narrow beam of light toward the side of the stone. A fiber-optic light works best for this type of lighting. You can aim the light straight at the stone or from an oblique angle. When you look at the stone from above, pinpoint crystals and gas bubbles stand out as bright objects. 19
GEM IDENTIFICATION
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Both by Eric Welch/GIA
With the light well’s baffle open, brightfield illumination transmits light up through a transparent stone to your eye.
You create brightfield pinpoint illumination by keeping the baffle open and closing the iris diaphragm until its opening is smaller than the stone.
Curved striae—Curved growth patterns seen in flame-fusion synthetics.
John Koivula/GIA
Brightfield pinpoint illumination reveals gas bubbles and curved striae in a flamefusion synthetic ruby. BRIGHTFIELD ILLUMINATION
Brightfield illumination—sometimes called transmitted light—results when you open the light well’s baffle so the light is transmitted directly through the stone to your eye. To keep from being dazzled by the bright light, close the iris diaphragm so the opening is smaller than the stone. This will create focused, pinpoint illumination. If necessary, adjust the intensity of the light source with the rheostat. Brightfield illumination makes inclusions look dark and featureless against a bright background, so it works well for seeing low-relief features like curved striae in flame-fusion synthetics. Curved striae are structures that represent the layers of crystal growth around the 20
MAGNIFICATION
Use brightfield illumination to detect lowrelief features like curved striae.
Both by Terri Weimer/GIA
By rocking and tilting a stone in darkfield lighting, you can create a brightfield effect. Alternating dark to light can help you detect flash-effect colors in fillers or see if an inclusion is liquid or solid, transparent or opaque.
Both by John Koivula/GIA
Rocking and tilting this emerald to alternate dark (above) and bright (right) backgrounds revealed an orangy yellow to blue flash effect in the filler.
cylindrical or rod-shaped boule, which is a typical product of the flamefusion process. The brightfield technique works best if you close the iris diaphragm and restrict the light source to a small opening directly under the stone. This lets you see structures like curved striae more clearly. You can actually create an effect similar to brightfield by rocking and tilting the gem under darkfield lighting to create alternating dark and bright backgrounds. This can be helpful for detecting flash-effect colors in fillers—seen mostly in fracture-filled diamond and emerald—or determining if an inclusion is liquid or solid, transparent or opaque. 21
GEM IDENTIFICATION
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Eric Welch/GIA
To create vertical overhead illumination with a stone like this coral cabochon, angle the stone so light strikes at about a 90º angle to its surface (above). Reflected overhead light and magnification reveal the characteristic straight, irregular, fibrous structure in this shell cameo (left). REFLECTED LIGHTING
Darkfield, brightfield, and diffused lighting make many inclusions easier to see, but reflected light works better for surface characteristics and some types of inclusions. To examine a gem’s surface characteristics, you need to position the light source, which is usually the microscope’s overhead light unit, so the light reflects off the gem. The light should strike the gem’s surface at close to a 90° angle—this is called vertical overhead illumination. Thin, flat inclusions—like the thin films seen in many rubies—are easiest to see when light reflects from their surfaces. But you can’t use vertical overhead illumination to see internal characteristics like these because reflections from the gem’s surface block your view of the stone’s interior. Instead, you need to use a light source—such as a fiberoptic light—to direct a narrow beam of light at the stone from an oblique angle. Light entering the stone from that angle reflects from internal fractures, cleavages, and fingerprints, and makes them much easier to see. DIFFUSED LIGHTING
Both by Eric Welch/GIA
You can create diffused light by opening the baffle and placing a tissue or a piece of translucent white plastic on the stage over the well (top). Diffused lighting reveals uneven color zoning in this diffusion-treated sapphire (bottom). 22
For diffused lighting, open the baffle and the iris diaphragm and cover the stage opening with a white, translucent material. You can use facial tissue or even the white plastic diffuser from the microscope’s overhead light source. Diffused light can help you detect liquid inclusions in natural gems. It’s especially good for detecting curved color banding in flamefusion synthetics. And it’s excellent for detecting uneven color zoning in diffusion-treated corundum, where surface-related color often stands out against the white background.
MAGNIFICATION
John Koivula/GIA
By revealing interference colors, polarized light can help you distinguish between included crystals and gas bubbles or gas-filled cavities, which show no color.
Liquid inclusions, curved color banding, and uneven color zoning show up best in diffused lighting.
Eric Welch/GIA
You can create polarized light with a microscope by opening the baffle, placing one polarizing filter over the light well, and holding another between the stone and the objectives. Rotate the handheld polarizing filter to cross the filters. POLARIZED LIGHTING
You can create polarized light by opening the microscope’s baffle and placing one polarizing filter over the light well and another between the stone and the objectives. You can hold the second filter or attach it to the objectives. Your microscope then functions as a magnifying polariscope. Use this type of lighting to distinguish included crystals from similarlooking gas bubbles or gas-filled cavities. Crystals might show interference colors and are often surrounded by halos caused by strain, while cavities or gas bubbles won’t have these features. 23
GEM IDENTIFICATION
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Both by Eric Welch/GIA
To examine the surface of a stone, turn the well light off and use the microscope’s overhead light. Rotate the stone to examine every side.
EXAMINING A STONE
Always start the examination process at 10X magnification.
As you become more comfortable with the microscope, the steps involved in examining a stone will become second nature to you. The first step is always to thoroughly clean the stone you’re about to examine. This is very important: It’s easy to mistake grease and dust on the gem’s surface for abrasions or even inclusions. It’s important to hold the stone steady. If you use a stoneholder, attach it to the stage and position the stone over the light well. If you hold the stone in tweezers, rest them gently on the side of the light well. Examine the stone thoroughly. If the gem is transparent to translucent, examine both its surface and its interior. As you examine the stone, record what you see on the Gem Identification worksheet. If possible, turn off the other lights in your area while you’re working. 1. Set the magnification to 10X. Always start at this magnification level. 2. Start with the well light turned off, and use the microscope’s overhead light to examine the gem’s surface. Position the light and hold the stone so light reflects from its facets. Look at the top and the bottom, then all the way around the sides. 3. If your stone is transparent to translucent, examine its interior next. Turn off the overhead light and turn on the microscope’s internal light source. Make sure the baffle is closed and the iris diaphragm is completely open.
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MAGNIFICATION
All by Eric Welch/GIA
To examine a stone’s interior, turn the well light on, close the baffle, and turn the overhead light off. Vary the focus between the stone’s upper and lower surfaces to thoroughly examine its interior. Turn the stone to several positions and repeat the process.
4. To examine the interior, start by focusing on the surface, then hold the stone still and move the focus down slowly until the back of the gem comes into focus. Slowly raise the focus back to the gem’s upper surface. 5. Repeat the process from the top, from the bottom, and from every side to make sure you view the interior of the stone from every possible viewing angle. 6. Switch to higher-power magnification to identify any characteristics you can’t see at 10X. This will also help you determine the nature of hardto-see characteristics. If you have a microscope with a zoom system, you’ll soon learn to move from low to high magnification with ease. 25
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Seeing Doubling
All by Terri Weimer/GIA
If you look through a DR stone in an optic axis direction, you won’t see doubling (left). As you rotate the stone farther away from the optic axis, the doubling keeps increasing (center and right).
The splitting of light in a doubly refractive (DR) gem produces doubling: doubled images of facet junctions, inclusions, and other characteristics. To see doubling, you must look at the object through the stone—but not in an optic axis direction. Although birefringence, which is the cause of doubling, is a very constant property, the amount of doubling you see varies with the stone’s size. The larger the stone, the more doubling you’ll see. It also depends on your viewing angle when you observe the stone. Doubling is hard to see in gems like quartz and corundum, but readily visible in calcite, synthetic moissanite, and synthetic rutile. It’s especially useful for proving that over-the-limits stones like zircon are DR. With practice, you can even estimate birefringence by judging the separation between the doubled images. When you look for doubling:
• Always use the same power, such as 10X or 20X. • Look through the stone to the opposite side. Look for doubled images of facet junctions, inclusions, and scratches. Make sure that an image isn’t just a reflection. (This can occur close to facet junctions.)
• Look in at least three different directions to make sure you’re not looking down an optic axis. The strength of the doubling also varies with direction. Estimate birefringence in the direction of greatest doubling.
• To confirm doubling with a microscope, hold a polarizing filter between the stone and the microscope’s objectives. When you rotate the filter back and forth about 90º, the doubling appears and disappears. 26
MAGNIFICATION
Both by Terri Weimer/GIA
You might see doubling in some transparent DR stones when you examine them in darkfield lighting (left). You can confirm doubling by holding a polarizer between the stone and the objectives and rotating it, making the doubling appear and disappear (right).
Taijin Lu/GIA
When you look at a synthetic moissanite under magnification, you see double images of its facet junctions (20X).
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Both by Nicholas DelRe/GIA
Low magnification is best for detecting overall patterns of clarity characteristics (above), while high magnification is best for close examination of individual characteristics (left).
Low power is best for detecting overall patterns of clarity characteristics, while high power is best for identifying and examining individual characteristics in detail. Use only as much magnification as you need to identify an inclusion. If you magnify the feature too much, you might miss important patterns that would be more obvious at lower power. Now that you’ve been through the basic steps of examining gems with both the loupe and the microscope, it’s time to identify the basic characteristics you’re looking for in natural and synthetic gemstones as well as imitations.
IDENTIFYING CLARITY CHARACTERISTICS ■ Which inclusions are typical of natural gems? ■ Which characteristics are typical of synthetic and
imitation gems? ■ How can the microscope help you identify assembled
stones?
Clarity characteristics are often the key to a gem’s identity. They can help separate a gem that grew naturally in the earth from one grown synthetically in a laboratory. In rare cases, inclusions can even help establish the geographic origin of an important natural ruby, sapphire, or emerald. They can also usually tell you if a gem has been treated. Later, in the separation assignments, you’ll read much more about the inclusions that identify specific gems. This section is a brief overview to remind you of the various types of characteristics you’ll encounter, starting at the gem’s surface. 28
MAGNIFICATION
Alan Jobbins
Gems that grow in the earth often contain clarity characteristics—such as mineral crystals and needles—that you won’t see in synthetic stones. This helps you separate natural gems from synthetics.
John Koivula/GIA
A gold or platinum platelet is a telltale sign of a hydrothermal or flux synthetic gem. The environments these synthetics grow in often contain those metals, and microscopic remnants end up in the stones.
SURFACE CHARACTERISTICS
As you’ve learned, you use overhead, reflected light to examine a stone’s surface. Pay attention to areas that are vulnerable to damage, like the girdle and culet. You can hold large stones with tweezers or your fingers. Hold small stones with tweezers. With a loupe, look for blemishes like scratches and abrasions that indicate a stone’s hardness. Polishing wheels tend to 29
GEM IDENTIFICATION
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The amount of wear a gemstone shows can indicate its hardness.
Maha Tannous/GIA
Characteristics like melted surfaces are evidence of heat treatment in corundum.
Terri Weimer/GIA
Rounded facet junctions and the orange-peel effect are signs of molded glass.
round the facet edges of gems with hardness below Mohs 7. If a gem’s hardness is below Mohs 8 and it has been worn for any length of time, its facet edges will probably be abraded. Heat treatment leaves gems like colorless or blue zircon and tanzanite slightly brittle, so these gems often have abraded facet junctions. Heattreated corundum often has melted surfaces along the girdle and in other areas. Glass and plastic imitations are often shaped in molds, resulting in rounded facet junctions. As molten glass cools, it shrinks slightly, often producing concave facets with surface texture that gemologists call the “orange-peel” effect. Fractures that reach the surface can sometimes help you identify a gem. The surfaces of fractures in aggregates often have a granular, rough, or 30
MAGNIFICATION
Is the Characteristic Internal or External? There are three methods you can use to distinguish between a transparent gem’s external and internal characteristics. With a microscope, it’s best to use fairly high magnification—30X to 50X. REFLECTED LIGHT
Reflected light is the best and most widely used method. Hold the stone so light reflects from the surface the object appears to be on. If the object is external, such as a piece of dust on a facet, it will stand out. If it’s internal, you’ll see an unbroken, mirror-like reflection from the facet. PLANE OF FOCUS
Hold the stone so you’re looking at the surface the object appears to be on. Try to bring both the surface and the object into the sharpest possible focus. If both come into focus at the same time, the object is on the surface or extremely close to it. If the surface comes into focus first, the object is inside the gem. ROCKING
Choose a nearby feature you know is external—such as a facet junction or a scratch—to use as a reference. Then rock the stone slowly back and forth while you watch the object in question and the reference feature. If the object is on the same surface as the reference, it will move the same amount. If it’s within the gem, the object will move less.
Peter Johnston/GIA
When you use the plane-of-focus technique, you know an object is on the facet surface when the object and the surface are in focus at the same time (top). If the object is below the surface, it’s out of focus when the facet surface is in focus (center), and in focus when the facet surface is out of focus (bottom).
irregular texture, like the surface of a sugar cube. Fibrous materials like tiger’s-eye quartz can have splintery fracture surfaces. Most transparent gems, such as quartz, beryl, corundum, and tourmaline, have a conchoidal, or shell-like, fracture surface. If the stone has cleavage, you might see either flat cleavage faces or small conchoidal fractures alternating with flat cleavages, creating a step-like appearance. Surface-reaching fractures can contain some of the oil or dye used to conceal the fractures or to enhance the gem’s color. You can locate them by examining the gem’s surface in reflected light. Once you’ve found a surface-reaching fracture, switch to darkfield illumination to follow the fracture as it extends into the gem and look for signs of a filling material.
John Koivula/GIA
Reflected light is best for finding surfacereaching fractures like this one. It’s been filled, which makes its length difficult to determine. Switching to darkfield would help you detect the filling.
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Edward Gübelin/GIA
Needle-like mineral inclusions in emerald (above) and ruby (left) mean these gems are natural. The group of intersecting needles in the ruby is called silk.
INCLUSIONS IN NATURAL GEMS
Natural gems often have an abundance of clarity characteristics. In general, natural gems contain a far greater range of characteristics than synthetic ones.
Kari A. Kinnunen
Blocked crystal growth causes hollow or filled growth tubes in beryl.
Needle—A long, thin inclusion that can be a solid crystal or a hollow tube that might be filled with liquid or gas. Silk—Group of fine, needle-like inclusions. Two-phase inclusion—A cavity in a gem filled with a liquid and, typically, a gas. Three-phase inclusion—A cavity in a gem filled with a liquid, a gas, and one or more crystals. 32
Natural gems grow in an environment—the earth’s rocks—where they’re in competition with many other minerals for the ingredients of crystal growth. As they grow, they often trap other minerals as included crystals. By comparison, synthetic and imitation stones grow in much more controlled surroundings—the pristine environment of the laboratory or factory. As a result, there’s less opportunity for them to acquire foreign materials as they grow. This means that when you see a range of different mineral inclusions, you know you’re looking at a natural gem. A natural ruby, for example, might contain a variety of included mineral crystals. You might see colorless, rounded calcite, zircon, or apatite crystals and dense patterns of tiny, slender rutile needles that intersect to form silk. Needles can be solid or hollow. If they’re hollow, they might be filled with liquid or gas. Solid needles occur in corundum, garnet, and some emeralds. Hollow needles are frequent features of chrysoberyl. In tourmaline and beryl, the hollow needles are called growth tubes. They’re often much coarser than hollow needles in other gems, and might be capped by tiny included crystals. Gems that grow in mineral-rich, watery solutions often contain liquid inclusions. Topaz, beryl, and quartz can have abundant liquid inclusions. Sometimes an inclusion also contains a gas or a solid, or both. When only two of those things are present—a liquid and, typically, a gas—it’s a two-phase inclusion. If all three are present, it’s a three-phase inclusion.
MAGNIFICATION
Natural gems typically contain a far greater variety of inclusions than synthetic gems.
John Koivula/GIA
Three-phase inclusions are evidence of naturally formed emeralds. They contain a liquid, a solid, and a gas.
Negative crystal—An angular, hollow space within a gem that resembles a mineral inclusion.
Eduard Gübelin/GIA
Negative crystals are angular, hollow spaces that usually contain a liquid and a gas.
Sometimes, a gem might contain angular spaces that adopted the shape and symmetry of the enclosing gem crystal when it cooled. They look like mineral inclusions, but they’re not. These hollow areas are called negative crystals, and they usually contain a liquid or a gas, or both. Negative crystals are common in corundum, quartz, topaz, and beryl. They can also occur in synthetic gems, so you’ll have to look for other evidence to be sure the gem is natural. If you suspect you’re looking at a negative crystal, you can use polarized light to confirm it. Unlike a solid mineral crystal, a negative crystal shows no strain colors. 33
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Both by Mike Havstad/GIA
Michael Waitzman/GIA
Fingerprint inclusions can contain a variety of materials. The fingerprint in the spinel (above) contains crystals and negative crystals. The fingerprint in the first blue sapphire (top left) contains negative crystals and two-phase or liquid inclusions. The other blue sapphire (bottom left) has a fingerprint that’s composed of liquid within partially healed fractures.
Crystals often fracture during growth. Sometimes fluids seep into the breaks and become trapped as the fracture recrystallizes. If a break doesn’t heal completely, it creates a pattern of small, disconnected fluid inclusions within the stone. Because of its appearance, the inclusion is called a fingerprint. Fingerprints can also consist of included crystals, two-phase or three-phase inclusions, or negative crystals, as long as they form a fingerprint-like pattern.
Robert Kane/GIA
Distinctly bluish clouds often occur in Vietnamese rubies.
Inclusions can be so tiny and numerous that it’s hard to see them individually, even at the highest magnification. When they’re numerous enough, describe them as a cloud. A cloud is any hazy or milky area that can’t be described as a feather, fingerprint, or group of included crystals or needles. Many diamonds, rubies, and sapphires contain clouds.
Cloud—Any hazy or milky area that cannot be described as a feather, fingerprint, or group of included crystals or needles.
As crystals grow, their growth stages often show up as color zoning. Color zoning is a pattern of alternating light and dark areas or of different colors. It’s often seen in gems like corundum, quartz, and tourmaline. It’s caused by variations in trace elements during crystal growth.
Fingerprint—Inclusions that form a pattern that often resembles a human fingerprint.
In natural gems, the bands are straight and angular, following the gem’s crystal structure. Synthetics can have straight or angular color zoning, which indicate flux and hydrothermal growth processes, or
34
MAGNIFICATION
Rolf Schwieger
John Koivula/GIA
Angular color zoning (above) and straight color zoning (right) often occur in blue sapphires. The zoning follows the gem’s crystal structure.
curved color zoning, which means they were produced by flame-fusion or pulling processes. INCLUSIONS IN SYNTHETICS AND IMITATIONS
The inclusions in synthetic gems often indicate the growth process the manufacturer used to produce them. You might see gas bubbles in synthetics produced by the flame-fusion or pulling processes. They’re especially likely in flame-fusion synthetics, where they can be spherical, elongated, or distorted. Spherical gas bubbles might have dark centers that make them look like doughnuts. The only untreated natural materials that contain gas bubbles are natural glasses like obsidian and moldavite and natural resins like amber. Gas bubbles occur in these natural amorphous materials, but almost never in natural crystalline materials, except as part of two-phase or three-phase inclusions or in the junction planes of assembled stones. They might also occur in glass or plastic fillers or where mineral inclusions were melted by heat treatment. Some natural gems can contain rounded crystals that resemble gas bubbles. Synthetics grown by the flux process usually contain inclusions that are remnants of the medium that the ingredients for crystal growth were dissolved in. Although they’re often thick and coarse looking, resembling
John Koivula/GIA
Gas bubbles are common in flamefusion synthetics like this manmade ruby.
The only untreated natural gem materials that contain gas bubbles are natural glasses and resins.
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GEM IDENTIFICATION
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John Koivula/GIA
Karl Schmetzer
The thick, coarse, grainy texture of this inclusion is typical of material trapped in synthetic ruby crystals as they grow by the flux process.
Some flux inclusions look delicate and are called wispy veils. These appear in a Russian synthetic alexandrite.
John Koivula/GIA
Duncan Pay
These large, white flux inclusions in a synthetic emerald are interconnected with smaller flux channels.
The appearance of included flux can vary according to the manufacturer’s process. These yellow-to-orange flux inclusions appear in a Ramaura synthetic ruby.
icicles, they can also be delicate in appearance. Gemologists often describe the delicate ones as “wispy veils.” They can resemble the fingerprints in natural gems, but are often folded and twisted, while fingerprints usually look flattened. Flux has higher relief than fingerprints, and it might contain trapped gas bubbles. It’s normally white, but it can be colorless, yellow, orange, or brown. As you read earlier, flame-fusion synthetics often show curved growth. Flame-fusion synthetic blue sapphires might show curved color banding. Unlike curved striae, the bands are different tones of the same color— often with alternating colorless bands—but they’re still curved. You’ll never see curved striae and curved color banding in natural crystals. John Koivula/GIA
Flame-fusion synthetic blue sapphires like this often show curved color banding. Here, it’s in the gem’s crown.
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Flux and hydrothermal synthetics can show straight or angular color zoning like that seen in natural material. The difference is that the zoning is generally more uniform in the synthetics than it is in natural stones. This is
MAGNIFICATION
Some hydrothermal synthetic emeralds can show liquid and two-phase inclusions and fingerprint-like patterns.
Flame-fusion synthetics might show curved striae or curved color banding, but natural gems never do.
John Koivula/GIA
In some hydrothermal synthetic emeralds, growth blockage can cause nailhead spicules to form. They’re usually near the seed plate and point away from it in the growth direction.
because of the more controlled environment the synthetics grow in. Some hydrothermal synthetic emeralds contain nailhead spicules, which are cone-shaped spaces extending from synthetic crystal inclusions. Modern heat-treatment techniques can alter many natural corundum inclusions so much that it can be difficult to separate them from some flux-grown synthetics. You’ll learn much more about the inclusions in synthetic stones in Assignment 11. IDENTIFYING ASSEMBLED STONES
You’ve learned that assembled stones are composed of two or more pieces of material glued or fused together to form one piece. In Assignment 2, you learned some ways to identify them. Typically, a 10X loupe is useful for detecting signs of assembly. It’s important to be careful, however, not
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GEM IDENTIFICATION
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Rene Moore/GIA
Heat treatment damaged this sapphire’s surface, forcing the cutter to repolish the gem. Repolishing added extra girdle planes. Don’t mistake them for the separation planes that indicate an assembled stone.
When inclusions that show in only natural or synthetic stones appear together in the same stone, you know it’s assembled. This sapphire-synthetic sapphire doublet shows straight growth and gas bubbles.
Curved color banding and a separation plane between the green crown and blue pavilion prove this is an assembled stone.
to confuse the multiple girdle planes that can occur in natural gems with the separation planes that indicate the presence of assembled parts. The ability to examine a gemstone under magnification is an absolutely indispensable skill for the gemologist. Now that you understand the basics, you should get as much practice as possible. In later assignments, as you test stones in your practice sets, take the time to look at each of them carefully. The gems in your practice sets can teach you a lot. In the future, something you see in these sets might help you identify a difficult gem. Even if you have access to a microscope, don’t rely on it exclusively. Practice with a loupe as well, because on buying trips, you’ll usually have to depend on it. In the next assignment, you’ll learn about the spectroscope. It’s another compact, portable instrument that you might want to take along with you. 38
MAGNIFICATION
Magnification is a valuable tool for detecting treatments and for separating natural gems from their synthetic counterparts.
Liquid inclusions, curved color banding, and uneven color zoning show up best in diffused lighting.
Practical experience and up-to-date knowledge are the keys to using magnification successfully in gem identification.
Always start the examination process at 10X magnification.
A fully corrected 10X triplet loupe is an essential gemological tool.
The amount of wear a gemstone shows can indicate its hardness.
Reflected light is best for examining a stone’s surface.
Natural gems typically contain a far greater variety of inclusions than synthetic gems.
Darkfield illumination works best for examining the interiors of transparent stones.
The only untreated natural gem materials that contain gas bubbles are natural glasses and resins.
Use brightfield illumination to detect low-relief features like curved striae.
Flame-fusion synthetics might show curved striae or curved color banding, but natural gems never do.
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key terms Baffle—A small metal plate that you can close to prevent the microscope’s light from shining directly through the stone from below. Base—The support platform that contains a microscope’s electrical controls. Blemish—Characteristic or irregularity confined to the surface of a polished gemstone. Chromatic aberration—Color distortion caused by the inability of a lens to bring the various colored wavelengths of light into focus at the same distance. Cloud—Any hazy or milky area that cannot be described as a feather, fingerprint, or group of included crystals or needles. Curved striae—Curved growth patterns seen in flame-fusion synthetics. Darkfield illumination—Lighting of a gemstone from the side against a black, non-reflective background. Depth of field—The distance that’s clear and sharp in front of and behind the point you focus on.
Light well—Housing for a microscope’s light source, located below the stage. Liquid inclusion—Pocket in a gem that’s filled with fluids and, sometimes, gas bubbles and crystals. Needle—A long, thin inclusion that can be a solid crystal or a hollow tube that might be filled with liquid or gas. Negative crystal—An angular, hollow space within a gem that resembles a mineral inclusion. Objectives—The lenses nearest the stone on a gemological microscope. Oculars—The eyepiece lenses on a gemological microscope. Pod—The housing for a microscope’s optical system, also called the head. Reflected lighting—Illumination of a gem’s surface by reflecting light from it. Relief—Contrast between an inclusion and its host gem.
Fingerprint—Inclusions that form a pattern that often resembles a human fingerprint.
Silk—Group of fine, needle-like inclusions.
Focal distance—The distance from the surface of a lens to a point that’s in sharp focus.
Spherical aberration—Blurring around the edges that occurs when a lens can’t get an entire image in focus at the same time.
Fully corrected triplet loupe—A loupe that contains a three-part lens that magnifies and corrects for spherical and chromatic aberration.
Stage—A microscope’s working platform.
Included crystal—A mineral crystal trapped within a gem as it grows. Inclusion—A characteristic enclosed within a gemstone or reaching its surface from the interior. Iris diaphragm—A device in the microscope’s stage that you can open or close to control the amount of light coming from the light well.
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Stoneholder—A device that attaches to a microscope’s stage to hold a gem steady. Three-phase inclusion—A cavity in a gem filled with a liquid, a gas, and one or more crystals. Two-phase inclusion—A cavity in a gem filled with a liquid and, typically, a gas.
MAGNIFICATION
ASSIGNMENT
6
QUESTIONNAIRE
Each of the questions or incomplete statements below is followed by several possible answers. Choose the ONE that BEST answers the question or completes the statement. Then place the letter (A, B, C, or D) corresponding to your answer in the blank at the left of the question. If you’re unsure about any question, go back, review the assignment, and find the correct answer. When you’ve answered all the questions, transfer your answers to the answer sheet.
________1.
Diffused lighting is most effective for detecting A. B. C. D.
________2.
A fringe of color that occurs when a lens focuses different wavelengths of light at different distances is a result of A. B. C. D.
________3.
astigmatism. full correction. spherical aberration. chromatic aberration.
The distance from the surface of a lens to a point that’s in sharp focus is called A. B. C. D.
________4.
thin, flat inclusions. high-relief inclusions. surface features like glass-filled cavities. curved color banding in flame-fusion synthetics.
relief. depth of field. focal distance. spherical aberration.
If a microscope’s oculars are 15X and the zoom adjustment is set at 2X, the magnification is A. B. C. D.
7X. 15X. 17X. 30X. CONTINUED NEXT PAGE...
IF YOU NEED HELP: Contact your instructor through GIA online, or call 800-421-7250 toll-free in the US and Canada, or 760-603-4000; after hours you can leave a message.
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GEM IDENTIFICATION
________5.
The amount of wear a gemstone shows can indicate its A. B. C. D.
________6.
cloud. needle. fingerprint. negative crystal.
A natural, untreated gem material that might contain gas bubbles is A. B. C. D.
________9.
a fracture. high relief. deep inside the stone. very near the surface.
A long, thin inclusion that can be a solid crystal or a hollow tube is called a A. B. C. D.
________8.
density. hardness. optic character. specific gravity.
Under the microscope, if the inclusion and the facet surface are both in focus at the same time, the inclusion is probably A. B. C. D.
________7.
6
ruby. peridot. obsidian. diamond.
The contrast between an inclusion and its host gem is called A. B. C. D.
relief. saturation. fluorescence. distinctiveness.
________10. The small metal flap that can be closed to prevent a microscope’s light from shining directly through the stone is called the A. B. C. D.
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pod. baffle. ocular. light port.
CONTINUED NEXT PAGE...
MAGNIFICATION
________11. To examine a gem’s surface, use A. B. C. D.
reflected lighting. darkfield lighting. monochromatic light. brightfield illumination.
________12. If you find curved striae or curved color banding in a gem, you know it is A. B. C. D.
natural. synthetic. heat treated. fracture filled.
________13. You can be sure that a gem is natural if it contains A. B. C. D.
a feather. wispy veils. straight color banding. a range of different mineral inclusions.
________14. Which of these would probably have concave facets? A. B. C. D.
Molded gems Enhanced gems Assembled gems Synthetic materials
________15. An angular, hollow space within a gem that resembles a mineral inclusion is called a A. B. C. D.
cloud. cavity. feather. negative crystal.
CONTINUED NEXT PAGE...
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Distance Education students: The following questions ask you to examine the stones in the set you’re currently working with. Choose the best answer to each question and continue filling in your answer sheet as you did with questions 1 through 15.
________16. Which of the following do you see in stone 4 using 10X magnification? A. B. C. D.
Doubling Wispy veils Gas bubbles Curved striae
________17. Which of the following do you see in stone 9 using 10X magnification? A. B. C. D.
Wispy veils Curved striae Included crystals Three-phase inclusions
________18. Which of the following do you see in stone 10 using 10X magnification? A. B. C. D.
Curved striae Flux inclusions Nailhead spicules Natural inclusions
________19. Which of the following do you see in stone 13 using 10X magnification? A. B. C. D.
Wispy veils Curved striae Included crystals Three-phase inclusions
________20. Which of the following do you see in stone 15 using 10X magnification? A. B. C. D.
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Wispy veils Negative crystals Included crystals Curved color banding
1. Introduction
12. Detecting Gem Treatments
2. General Observation
13. Separating Red, Pink, and Purple Gems
3. Refraction and the Refractometer 4. Polariscope Testing 5. Pleochroism and the Dichroscope 6. Magnification 7. Selective Absorption and the Spectroscope 8. Fluorescence and Phosphorescence 9. Additional Tests 10. Separation and Identification 11. Separating Natural Gems from Synthetics and Imitations
14. Separating Blue and Violet Gems 15. Separating Green Gems 16. Separating Orange, Yellow, and Brown Gems 17. Separating Colorless, White, Gray, and Black Gems 18. Identifying Rough Gems, Parcels, and Mounted Gems 19. Advanced Laboratory Testing
