CUTTING A GEM On this page you can watch the process of cutting a gem. I will go step by step, to the completed Gem. I have all ready decided what I want to cut, the cut is going to be a round amethyst with an apex cut crown and a brilliant bottom, there will be 16 bottom girdle facets, 8 bottom mains and 16 culet facets, and the crown will have 16 girdle crown facets with 3 rows of 8 crown mains. The size of the stone will depend upon the rough. A word here about cutting rough gems, for high value gems, diamonds, rubies, sapphires and emeralds the rough will in most all cases dictate the shape of the finished stone. The cutter in high value gems will always adhere to the general shape of the rough, buy doing so the cutter gets the maximum weight retention that the stone has to offer. In less costly gems the cutter has much more leeway in his decision as to what shape he may want to cut. However a good cutter wants to maximize as much weight retention as he can. REMEMBER WEIGHT IS MONEY. 1. THE ROUGH The first thing I am going to do is to select a suitable piece of rough. Some considerations here are color, realizing that amethyst can have strong color zones, clarity I want to have a clean finished stone and a piece of rough sufficiently large to produce a good size finished stone.
Which Stone? After looking over the parcel of rough I've selected the piece in the lower left corner, I felt this piece gives me the color clarity and size I want.
This is the rough I selected and what your looking at will be the apex table of the finished stone.
This is a side view of the rough, it weighs 39cts and measures 16.10mm to 18.80mm in diameter and 24.37mm long. The shape of this stone is not ideal, but I am willing to sacrifice some weight for the finished gem. 2. ROUGHING OUT The next step in the process is to remove some of the unwanted material. To do that I will take the rough to a diamond grinder and begin to shape the stone, this is called roughing out. The idea is to knock off some of the unwanted material, remove any inclusions and get the general shape of the stone.
This is the grinder I use in the roughing out process, The diamond wheel on the left is a used 100 grit which works well and will not leave heavy grinding marks. The deeper your grinding marks are on your stone the more work and greater weight loss there is in getting them out.
Here you see me cutting the temporary table of the stone. Every cutter has his own method as to how they like to cut a stone, some like cutting crown first and some the pavilion first. I prefer cutting the pavilion first, mainly because I can get a good hold on the table and center the stone easier when I dop table first.
Here are the results of roughing in the table, note the grinding marks and the slight curve to the table, which I will remove on a 600 grit flat lap. We are now down to 32ct weight. I will now continue to rough out the basic shape of the stone, I'll do this by grinding down the high sides of the stone.
Having knocked off the high sides of the stone I now have a good idea as to what size of finished stone I may be looking at. Right now the stone is over 15mm in all directions and weighs 27cts, as I am going to make this stone a standard size, I am looking at a 14mm finished gem. I will get the finished size on the faceting machine, once I have doped the stone. Do you know what a natural is and what is its importance in gemstone cutting? I've taken this stone as far as I can in the roughing out process, the next step is to flatten out the table and dop the stone so that it may begin the faceting process, the first step in that process is.... 3. THE DOPPING PROCESS Before I actually dop the stone, I am going to flatten of the table, I'll use a 600 grit flat lap on my faceting machine. Run plenty of water at 1/3rd speed and hold the stone steady, with a stone this size its easy enough to do by hand. On small stone you might have to dop the stone up to get the table cut.
What is dopping and how is it done? Dopping is the placing of the stone on a cutting stick known as a dop stick. The dop stick holds the stone in place so that it may be attached to the faceting machine, which is the machine that will cut and polish the facets on the gem. More about the faceting machine later. We have several kinds of dop sticks in a variety of sizes and shapes. Which dop we
will use depends upon the size of the stone, the shape and if we're cutting the crown or pavilion of the stone.
Here you see the flat dop on the left, a V dop in the middle and a cone dop on the end. Most cutters like using the cone dop when dopping to the flat of a stone, seems to get a better hold than the flat dop. When selecting the right size dop stick, you want one that is large enough to cover most of the dopping area and small enough not to interfere with the cutting.
Here I am beginning the dopping process, most cutters will use an alcohol lamp in the dopping process, (note alcohol flames are not very visible care must be taken when using alcohol) we heat the dop stick and apply dopping wax to the stick, it is this wax that will hold the stone.
I use green dopping wax to dop the stone with. Dopping wax is a shellac base wax which will melt at a low temperature, yet has a great holding ability.
Here I am heating the stone before I apply it to the dop stick. The secret to getting a good bond between your stone and the dop is temperature of the stone and wax. To cold not a good hold, to hot you cook your wax. After you've lost a few stones, you soon develop a feel for dopping.
The dopped stone, ready for the FACETING MACHINE. 4. THE FACETING MACHINE All faceting machines from the jam-peg to the automated machines, all work off a basic common denominator, and that is the ability to cut a facet at a particular angle and index and then be able to come back to the same facet and polish it at the same angle and index. The machine I cut with is a fixed mast unit, I have a number of machines, I am currently using a "Scintillator 88" manufactured by Poly-Metric Mfg. A word here about faceting machines, any machine that I have cut on has always had its own quirks. A good cutter knows what they are and will compensate for them. I like the saying "its a poor workman that blames his tools".
The "Scintillator 88" Starting at the left you have your speed and direction control, a storage container for water, your arbor for your laps, the quill & index assembly, a positive stop and the mast with LED angle read out. These are the basic components of the faceting machine. 5. Cutting the Stone I have the stone dopped, I like establishing my girdle first, so that is what I am going to cut. I place the dop stick with the stone in the end of the quill.
using positive stop".
"Dop stick in the quill"
"Grinding stone
I set my quill at 90° and adjust the stop by grinding two opposite sides of the stone to almost the finished size. Then I cut all the other facets to the stop at there appropriate indices. Remember this is going to be a 16 sided stone and I am using a 96 index gear. So that means that I am cutting a facet every 6th index.
girdle breaks at 63°"
"16 girdle facets"
"16
The cut girdle of the stone, which is just at 14mm in diameter. I will now cut the first girdle breaks at an angle of 63º using the same index setting as I did
on the girdle, I start my cutting at the 96 index notch. The cutting of the girdle breaks establish the finished girdle.
The 96 index wheel. The 96 tells the number of divisions on the 360° of a circle. We also have 32, 64, 80, and 120 index wheels available.
"8 mains at 55°"
"16 culet
facets at 43°" I am now going to cut a main facet between every other break facet, that will be 8 main facets at an angle 55° indexing every 12th notch on the 96 index wheel. Every facet that is cut on a faceting machine correlates to a number on the index wheel and an angle. I have also cut 16 culet facets, these were cut at the girdle index at an angle of 43°. I have now cut 56 facets on the stone 40 on the pavilion and 16 girdle facets. The next step in the cutting process is to go back and polish the stone. Before we get into the polishing of the stone, I need to tell you about the LAPS. 6. LAPS A cutter usually has a variety of cutting and polish laps, I have over 50 different laps myself, and there is a good reason to have everyone. CUTTING LAPS, they can be solid steel, aluminum backed, or copper, these laps are usually a diamond coated lap that comes in a variety of diamond grit size. You need a heavy (80-100) grit diamond lap to remove large amounts of material quickly. You need medium (180-260) grit to smooth out heavier cutting, or to work smaller stones. Your pre-polishing fine laps (600-1200) diamond grit will give you a surface that is ready to polish. Some cutter like taking there stone to 3000 diamond grit or more for pre-polish, its a cutters choice. POLISHING LAPS, every cutter has a large number of polishing laps, all kinds of metal (tin, type, copper, steel, babit,lead and combinations of metal) all used to get the perfect polish. Plastic laps ( lucite, phenolic, vinyl, and combinations like the last lap) Soft laps like wood and wax. You also have polishing compounds that you use in combination with your laps to achieve your polish. ( cerium oxide, alumina oxide,tin oxide,chrome oxide and diamond) I am not going into detail about the use in combination of lap, compound and material that will achieve the final polish, lets just say that depending upon the material your polishing will determine the lap and compound that you should or could use. 7. Polishing When polishing a cutter has options as to where to start the polishing process, most text on the subject like to polish back wards to the cutting process. starting with the culet facets first and going down
That's all for now will work the stone, show you the process and the results in the next installment.
