Thank you in advance. Kind regards from Istanbul,
To judge your yield from facetable material, you first have to determine how much of the stone is clean and cuttable. If it doesn’t need any trimming, that’s easy. Otherwise, you have to estimate how much weight will have to be removed to get a clean piece that you can dop and facet. I find it easiest to judge that as a percentage. For example, if I see a fracture that needs to be cut out, I’ll estimate how much the area to be removed will weigh. If that is 10% or 20% of the stone, I’ll then use a calculator to figure how much of the piece is suitable for faceting.
The yield from a clean, facetable piece will usually range between 25% and 33% with custom cutting and 15% to 25% with factory cutting. The more evenly shaped the rough, the higher the yield. If the stone has a lot of projections that need to be cut down, then the yield will be lower. If the stone is exceptionally well shaped, already tapered towards the culet, you might be able to get 40% or more.
The next determining factor for faceting yield is what size you need to cut the rough to. If you’re free to cut for maximum yield, you can expect 33% weight retention from a relatively chunky piece of rough. On the other hand, if you need to cut the stone down to fit a calibrated setting, your yield can be much lower. How much lower will depend on the individual piece of rough. For example, if the rough will allow a 9 by 5 mm oval, that would weigh close to 2 carats. If you need to cut it down to the closest calibrated size, that would be a 7 by 5 mm oval and would weigh closer to one carat. That is a major weight reduction. In this case, you should consult with your customer before proceeding.
If you’re cutting an amethyst for him, a custom-made setting to hold an unusual size may not be in his best interest financially. However, if the gem were valuable, it would be worth faceting for maximum yield and having the setting custom made. This is a pretty complicated subject, but I hope these guidelines help you.
Donald Clark, CSM IMG