Cutting a Gem

7/27/2019 Cutting a Gem

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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 roundamethyst with an apex cut crown and a brilliant bottom, there will be 16 bottomgirdle facets, 8 bottom mains and 16 culet facets, and the crown will have 16girdle crown facets with 3 rows of 8 crown mains. The size of the stone willdepend upon the rough. A word here about cutting rough gems, for high valuegems, diamonds, rubies, sapphires and emeralds the rough will in most all casesdictate the shape of the finished stone. The cutter in high value gems willalways 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 gemsthe cutter has much more leeway in his decision as to what shape he may want tocut. However a good cutter wants to maximize as much weight retention as hecan. REMEMBER WEIGHT IS MONEY.

1. THE ROUGH The first thing I am going to do is to select a suitable pieceof rough. Some considerations here are color, realizing that amethyst can havestrong color zones, clarity I want to have a clean finished stone and a piece ofrough 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 leftcorner, 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 ofthe finished stone.



This is a side view of the rough, it weighs 39cts and measures 16.10mm to18.80mm in diameter and 24.37mm long. The shape of this stone is not ideal, butI am willing to sacrifice some weight for the finished gem.

2. ROUGHING OUT The next step in the process is to remove some of theunwanted material. To do that I will take the rough to a diamond grinder andbegin to shape the stone, this is called roughing out. The idea is to knock offsome of the unwanted material, remove any inclusions and get the general shapeof the stone.



This is the grinder I use in the roughing out process, The diamond wheel on theleft 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 greaterweight loss there is in getting them out.



Here you see me cutting the temporary table of the stone. Every cutter has hisown method as to how they like to cut a stone, some like cutting crown first andsome the pavilion first. I prefer cutting the pavilion first, mainly because I canget 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 theslight curve to the table, which I will remove on a 600 grit flat lap. We are nowdown to 32ct weight. I will now continue to rough out the basic shape of thestone, 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 towhat size of finished stone I may be looking at. Right now the stone is over15mm in all directions and weighs 27cts, as I am going to make this stone astandard 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 stepis to flatten out the table and dop the stone so that it may begin the facetingprocess, the first step in that process is....

3. THE DOPPING PROCESS Before I actually dop the stone, I am going toflatten of the table, I'll use a 600 grit flat lap on my faceting machine. Runplenty of water at 1/3rd speed and hold the stone steady, with a stone this sizeits easy enough to do by hand. On small stone you might have to dop the stoneup to get the table cut.

What is dopping and how is it done? Dopping is the placing of the stone on acutting stick known as a dop stick. The dop stick holds the stone in place so thatit may be attached to the faceting machine, which is the machine that will cutand 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 thecrown or pavilion of the stone.

Here you see the flat dop on the left, a V dop in the middle and a cone dop onthe end. Most cutters like using the cone dop when dopping to the flat of astone, seems to get a better hold than the flat dop. When selecting the rightsize 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 inthe dopping process, (note alcohol flames are not very visible care must be takenwhen using alcohol) we heat the dop stick and apply dopping wax to the stick, itis this wax that will hold the stone.



I use green dopping wax to dop the stone with. Dopping wax is a shellac basewax 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 stoneand wax. To cold not a good hold, to hot you cook your wax. After you've lost afew 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 theautomated 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 comeback to the same facet and polish it at the same angle and index. The machineI cut with is a fixed mast unit, I have a number of machines, I am currentlyusing a "Scintillator 88" manufactured by Poly-Metric Mfg. A word here aboutfaceting 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 thesaying "its a poor workman that blames his tools".



The "Scintillator 88"

Starting at the left you have your speed and direction control, a storagecontainer 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 basiccomponents of the faceting machine.

5. Cutting the Stone I have the stone dopped, I like establishing my girdlefirst, so that is what I am going to cut. I place the dop stick with the stone inthe end of the quill.



"Dop stick in the quill" "Grinding stoneusing positive stop".



I set my quill at 90° and adjust the stop by grinding two opposite sides of thestone to almost the finished size. Then I cut all the other facets to the stop atthere appropriate indices. Remember this is going to be a 16 sided stone and Iam using a 96 index gear. So that means that I am cutting a facet every 6th

index.



"16 girdle facets" "16girdle breaks at 63°"

The cut girdle of the stone, which is just at 14mm in diameter. I will now cutthe 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 thegirdle 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 culetfacets 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 theindex wheel and an angle. I have also cut 16 culet facets, these were cut at thegirdle index at an angle of 43°. I have now cut 56 facets on the stone 40 on thepavilion and 16 girdle facets. The next step in the cutting process is to go backand polish the stone. Before we get into the polishing of the stone, I need totell 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 lapsare 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 materialquickly. You need medium (180-260) grit to smooth out heavier cutting, or towork smaller stones. Your pre-polishing fine laps (600-1200) diamond grit willgive you a surface that is ready to polish. Some cutter like taking there stoneto 3000 diamond grit or more for pre-polish, its a cutters choice. POLISHINGLAPS, 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 theperfect polish. Plastic laps ( lucite, phenolic, vinyl, and combinations like the lastlap) Soft laps like wood and wax. You also have polishing compounds that youuse in combination with your laps to achieve your polish. ( cerium oxide, aluminaoxide,tin oxide,chrome oxide and diamond) I am not going into detail about theuse in combination of lap, compound and material that will achieve the finalpolish, lets just say that depending upon the material your polishing willdetermine the lap and compound that you should or could use.

7. Polishing When polishing a cutter has options as to where to start thepolishing process, most text on the subject like to polish back wards to thecutting 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 inthe next installment.

Cutting a Gem

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