Optical Axis Tips
by Charles W. Covill
I recently bought several thousand carats of golden beryl (heliodor) from a Russian dealer. The material is from Tajikistan, Russia. The local area is known as “Zelatovia Vada” or yellow water. This background information is just to establish the material for you. Almost all of the pieces are crystal sections and are nearly flawless. The pieces in this parcel range in size from 1/4 inch to over 1/2 inch in diameter and from 1/4 inch to over 3 inches long.
This gem material is absolutely beautiful, and I have cut several stones from this material using the same diagram. I cut the stones with the table orientated on the A axis, the B axis, and the C axis. The stones cut with the table on the sides of the crystal (the A axis and the B axis) are nice and quite beautiful. However, the stones cut on the C axis are exceptional! Cut on the C axis, the color is richer and gives the stone more brilliance.
I have cut similar stones from other materials, such as quartz, aquamarine, and tourmaline, just to name a few. In all cases, the stones with the table set perpendicular to the C axis are superior to the ones cut with the table perpendicular to the A axis and the B axis. I first noticed this when cutting topaz many years ago. Stones with the table close to the cleavage plane show a much better optical performance, and the color is greatly enhanced.
With a crystal section, it is easy to locate the C axis or optical axis. The C axis runs from one end of the crystal to the other. In most cases, if the crystal is clear, and you can see through it, I recommend that you buy gem rough by looking into the ends. Your stones will be more lively and more brilliant cut on the C axis, and the color will be intensified when you place the table perpendicular to the C axis.
Orientating the stone is easy if you have a crystal section, but what about material that is not in an obvious crystal shape. The same properties hold true, as the A axis, the B axis, and the C axis are all present: However, you may need to use a polariscope to find the C axis in some cases.
This true story will illustrate the advantages I have described and will further reinforce my point. A student from one of my classes in Austin, Texas faceted a danburite along the C axis using one of my tourmaline crystal cut faceting designs. He set the stone in a gold pendant mounting and gave it to his wife. She proudly wore it to work one day. A man who she was talking to noticed the pendant and asked her what it was. She replied, “It’s danburite.” He then said, “I beg your pardon?” She repeated, “It’s danburite.” The man exclaimed, “I know its damn bright, but what is it”!?
I would like more input on this thread of evidence. How to make and use a polariscope needs to be covered and properly explained. Dichroism also enters into this. I have merely scratched the surface here, but much more needs to be said regarding the subject of orientating your gem rough.