What is all of this talk about preforming a piece of rough gem material? Why would you preform using a grinder? These and other suggestions are often made in descriptions on how to proceed to facet a gemstone.
I see no benefit from preforming. I do see how a lot of good material might be lost in overcutting, while being in the hurry to reduce the size or shape of a piece of rough. Why not start from the beginning with the idea of cutting with precision? This is so that you are left with the largest possible size for the design that you are trying to complete.
A preform gives you nothing but a poorly shaped piece, that will have more areas removed as you achieve the desired finished shape. To suggest that the preforming be done using a grinder just boggles my mind. This certainly should not be done with gem materials that are very sensitive to heat and to rough treatment…like Kunzite! You are bound to introduce shock, especially to some of the delicate materials. The effects will show up later, as you work on the gem. Kunzite can simply fall apart on you, unexpectedly, like layers of rock being shaken by an earthquake!
I do not preform stones: that is, unless you call using the gem saw to cut a piece off of a large hunk of gem material as doing preforming. I go to work faceting this resulting square or rectangular piece of rough with the angles and indexes called out for in the design. I work with certain facets in the design, which will get rid of some undesired material, so that I can do the final job sooner, but this choice of certain facets is never the same. I just decide what is best with each stone and design.
When you want to cut a stone to the largest possible size that can be done, there is always some point when you find a certain facet that will not allow more cutting on it. That facet determines the size that you can achieve. You certainly do not want to exceed that facet because you used too much speed and laps of too coarse grit! That is what preforming will get you…a cut too far.
Do not tell me that you want to save all of the little pieces of gem material, and so you use the saw to preform the stone. You will not find much use for those little pieces, and they will just cut your fingers because of their sharp edges. Let’s face it. Your finished stone will only be 20% to 25% of the weight of the rough you started with. You are almost certain to over-cut if you use the saw!
If the rough is large, and requires removal of a lot of material before getting near the desired finished size, I start with a 220 grit diamond bonded lap, which is quite worn through years of use. It is not real sharp anymore. With this lap I put in only enough facets to get rid of the bulk of the unwanted material. Even this lap can introduce strain in some materials. It can come back to haunt you later, as you proceed with the gemcut.
I guess some would call using the 220 lap to shape the stone as doing preforming. I do not.
After using the 220 (if I really did) I use a 325 Crystallite metalbond lap, which cuts quite fast, and is not so rough on the rough. I often use the 325 as the first lap to touch the gem. The 325 is an expensive lap, but well worth its cost. With the 325 I cut the gem down to near the desired finished shape and size. I put in all of the major facets of the design, but leave the small or delicate facets for the finer laps.
Next, I usually use my Maja 1200 diamond bonded lap, to do all of the facets in the design. This time I am being careful to get all of the facts as near perfect as I can. Very little material will have to be removed in further faceting.
My last cutting lap or pre-polish lap if you want to call it that, is a copper lap. I bought this lap when I started faceting, many years ago. I recharge this lap with 1200-mesh diamond compound from a syringe. I force the diamond into the copper with a metal roller. I seldom do this recharging, because I want the lap to be well worn and to have all of the larger pieces of diamond broken down. Then, I do not get scratched facets. This lap is actually a much smoother and slower cutting lap than the Maja 1200 diamond bonded lap described earlier. I use this lap far more than any of my other laps. Most of my cutting time is spent with this lap. With it, I put in the final facets. With it, I get my meets as accurate as I am capable of achieving. I do not expect to polish the meets into the proper places.
There are enough problems with the polishing process. Do you need the added burden of shaping facets and meets? No, the idea is to have very smooth facets after your cutting is completed. You want facets that have perfect meets and are ready for polishing.
Sometimes I go over the cut facets with my worn out 3000 lap. This lap leaves a surface that is almost polished.
Now comes the really hard part…the polishing of your gem material. You have to experiment to find what works for you. What works for you might also work for someone else, but often it does not. Sometimes your method will not work for you the next time. But keep notes on the materials you work with. Record the type of lap used, speeds used, polish powder used, etc. Also record the problems you experienced with the use of the wrong laps and polish powders.
Polishing is what makes the finished gem a winner or a disaster. Not just the quality of the polished surface, but whether it is a good surface while having the facets meet at the proper places. You have to spend the required time. Patience will get those finished meets where they belong, along with suitable flat polished surfaces.
So from the beginning, think perfect cutting and think perfect polishing. Do not think fast cutting and rough shaping. Do think of being a performer…not a preformer.