How to Obtain the Best Yield by Wayne S. Barnett
One of the goals of most faceters is to obtain the best yield from the rough that they are cutting. Unfortunately many faceters blindly go into cutting a stone and do not plan on the best approach. That is perhaps just human nature. While I have not run any hard statistics on the stones that I have cut, the numbers that I have gleaned from various sources indicate that the best most rough will yield is about 30 percent. If you were to send a couple of kilos to the Mideast cutting shops you might get about 10 percent. Not very encouraging, considering the amount of time and money that would go into a stone.
So how do we optimize the yield from the material we cut. Since many, if not most, of us cut only a few stones a year and these are from material that we cherry pick from a dealers stock, we can try to eliminate many of the factors that lead to a poor yield. First do not even consider material that does not have relatively large areas of clean stone. If there are fractures or undesirable inclusions in the rough make sure that they are in area that will be cut away or will not show in the final stone. When purchasing material in person use a small flashlight to look at the interior of each piece. Turn the piece at every angle to try and see any veils or fractures that may be in the piece. If it is fractured and the piece cannot be trimmed into sections with clean areas, then reject it.
Once the piece has been determined to be clean or can be subdivided into two or more clean stones then the shape needs to be evaluated. Will this rough approximate the shape of the stone that I wish to cut? The closer the rough is to the final shape of the gem that is required the greater the yield. If material that is misshapen in shape is chosen the yield will be considerably less that optimum. Sometimes this cannot be avoided.
When stones are cut and the critical angles are honored the stone will have a predictable total height to total width. In general as the refractive index increases the total depth decreases. Diamond, being the most commonly commercially mounted stone, has most of the ready made mountings made to fit them. Since diamonds have one of the highest refractive indexes for gemstones it will be cut into stones that are more shallow than most colored stones. This makes finding mountings for colored stones more difficult if they have been cut to the proper angles or they are cut with designs that are deeper than normal. In general the higher the refractive index the shallower the finished stone (if the critical angles are honored).
The width and depth of the rough will determine the largest size that the stone can be cut. While this may be self-evident, a large flat piece of rough may not cut as large a stone as a smaller well-shaped piece of rough because of its shallow depth. Depending on the material and the cut, the depth of the stone is usually 60 - 80% of the width of the stone. So, in addition to getting a piece of rough that is wide enough for a given size stone the depth must also be considered.
Normally the depth of the stone should be about 70% of the required width. Alternately if the depth is the limiting factor the depth times about 1.4 will give the maximum width of stone that can be cut without violating the critical angles. Even so, there may be certain novelty cuts where you may want to ignore the critical angles or traditional proportions. In those cases the rules can be ignored and the stone cut to achieve the desired results.
Once the rough has been chosen for a given stone the quest for the greatest yield is not over. This is where the art of the cutter takes over. The first step in the process is to dop the rough on the dop in the best position so that the center of the finished stone is over the center of the end of the dop. See the article on dopping elsewhere on this web page. Once this is done the cutting can begin. Put the dop in the quill of the facet machine. Make sure that it is oriented properly so that the beginning index is oriented properly in relation the shape of the stone to be cut. Tighten it down and cutting can begin. Begin cutting the set of facets that will form the culet of the stone. Adjust the head above the lap so that the stone barely touches the lap on the side that is the shallowest or that will require the least cutting before the stop is reached. This is the limiting cut. Cut this facet to the stop then turn the stone 180 degrees (assuming a design of even number of cuts around the stone) to cut the facet on the opposite side. Once this is done there should be a line where the two meet or an area that has not been cut and rough in between the two cuts. In either case do not lower the head any more until the rest of the cuts in that set are made around the stone.
Once all of the first cuts are made around the stone the shape of the pavilion should be established and a small flat or rough area will be where the culet will be established. If the pavilion comes to a point at this stage the facet head may have been lowered to much for the first set of cuts. Next set the head of the faceter to cut the girdle. This step will allow you to rough in the shape of the final stone. Depending on the size of the stone and the amount of height, cut only enough to roughly establish the shape of the girdle. If you leave some rough areas at this point that is OK because you will be coming back to finish the girdle with a fine cutting after the final shaping of the pavilion with finer laps.
The final depth of the pavilion now needs to be established. Using a fine lap begin recutting the facets that established the shape of the pavilion by positioning the head of the faceter above the lap so that the stone is barely touching the lap. Cut the first facet then again turn the quill 180 degrees to cut the opposite side. If the two do not meet this time lower the head a little and cut to the stop on each side until they meet. Once they meet or barely not meet cut the remainder of the facets for this set. If this is the final cut before the polish, which it may be for smaller stones, cut the set so that the point at the culet is established and all facets meet at that point. This will establish the final shape of the pavilion.
Depending on the cut being made the next step will be to either cut the girdle break facets or the girdle itself. Adjust the head so that the stone will barely touch the lap once the angle has been set. Cut with a fine lap around the stone and establish the depth as two or three cuts are made, again cutting to the stop and gradually lowering the head until the correct depth is established. As each set is cut make the points meet, even if the final depth of cut has not been established. After all the cuts have been made polish the pavilion. Do not polish the girdle.
Transfer the stone to cut the crown. Put the stone in the faceter quill and bring the head down so the quill can be set to about 90 degrees. Set the index at one of the indexes that the stone shape was established and hold it flat to the lap. With the stone being held to the lap on one of the flat girdle facets tighten the dop in the quill. Raise the head to cut the mains of the crown and proceed as on the pavilion, cutting a little at a time until the shape of the crown is established and the thickness of the girdle is established plus a little bit. The girdle needs to be a little thicker at this point because the final finishing of the stone will remove some of the girdle thickness.
Bring the faceter head down so that the quill can be set to 90 degrees. Put the polish lap on and polish the girdle. Begin by polishing a little bit and then inspecting to see where it was polished. Adjust the rotation cheater and the height cheater so that the girdle face is being evenly polished. This may take two or three facets to finally establish the absolute best fit. Once established finish polishing the girdle. Make special note of the rotation cheater setting and make sure that it remains the same for the rest of the cutting. This is the zero setting to match the crown to the girdle. Finish the crown cutting each set of facets in stages until the proper depth is established.
Some common errors that reduce yield of rough are:
Not being careful on how a piece of rough is being preshaped with a saw or grinding wheel.
Using a thick saw blade instead of a thin gem blade.
Grinding to deep on the first cuts to the stone.
Not properly orienting the stone on the dop in regard to cleavage, a particular issue when cutting topaz, and having to recut the stone.
Not properly centering the rough on the dop in relation to the center of the finished stone.
Not finding flaws in the rough until cutting has begun.
Making errors in the index or angle settings while the stone is being cut.
Pressing down and over cutting a facet.
Trying to do to much to fast.