(reprinted from the December 1998 issue of the USFG Newsletter with permission from the USFG)
This article is reprinted here with permission of the Columbia-Willamette Faceters’ Guild of Portland, Oregon. It was published in the June, 1976 issue of FACETS,the bulletin of the C.W.F.G. It shows how a little knowledge of gemology and crystallography can help factors turn out more beautiful gemstones.
An experiment recommended some years back in the Lapidary Journal illustrates an important consideration in orienting rough faceting material, in order to obtain the brightest finished stones possible. The intent of the experiment was to point out the differences in brilliance which may occur as a result of orienting two separate stones differently with respect to the optic axis within the same crystal. Aside from your faceting equipment, all you will need is a single crystal large enough to cut two stones, and some means of sawing it in two.
Because of some degree of dichroism which exists in most colored rough, it is recommended that the material used for this experiment be colorless. Quartz is an excellent choice, and so is goshenite (the colorless form of beryl) –but the crystal habit of the material, whatever you might use, must be apparent.
Be sure to use one single crystal and cut two stones from it. Otherwise, after they’re cut, there might be the suspicion that a difference in the material of the two stones existed. First, compare your crystal to the drawing shown, and visualize the two stones in it, so you can place the saw cut properly with respect to the optic axis. This optic axis is an imaginary line running the long way through the center of the crystal. Be sure to have one stone oriented with its table at right angles to the optic axis, while keeping the other stone’s table parallel to it.
The stone with the right-angle orientation can have its table face either directly toward the terminated end or directly opposite. Either way will do, as long as the completed stone will have the viewer looking along the axis as he looks down at the top of the stone (see perspective A).
The parallel-oriented stone actually may be cut at many different angles. The important thing is to NOT use the flat sawed surface for a table, nor the other end of the crystal. The easiest way is to just use one of the six crystal sides as a table. Thus, you will end up with a finished stone having the optic axis running through it from side to side as you look down on the table, as in perspective B.
When faceting, almost any cut will do. The standard round brilliant is fine. But it is very important that you use exactly the same cut for both stones, and exactly the same angles. There is a strong temptation at this point to try two different cuts. This would ruin the experiment. The difference in the twin stones is so drastic, it would forever be blamed on the difference in cuts. So make two stones as exactly alike as you possibly can.
Now, inspect the two stones together under a strong light, preferably on a black background, with both sparkling clean. Is one dull, lifeless, and actually glasslike? Is the other sparkling, vibrant with life, and truly a gem? Do you know which is which? Do you know why?