Hi all, I purchased some peridot rough at Gemfest and have just faceted my first piece. I used angles typically used for sapphires (higher RI) and ended up with a black dot in the centre of the stone, my first fisheye. So I played around with SRBs on Gemcad and came up with a design that looked good. So I recut the stone but now get a dark shadow towards the girdle when the stone is tilted. Has anyone else had this problem? Anything to do with birefringence? Cheers - John
It seems you have cut it at the wrong angles
I would recommend you purchase the Book Cutting Gemstones from the AFG supplies
Thanks Edward, I have that book - maybe I should re read it. But I thought I had a good design using Gemcad. I will have another go with a second piece that I have. I have only cut high RI material before (CZ, Sapphire and Zircon) this is my first foray into lower RI stuff and I have plenty of Quartz and Topaz to work with yet. Cheers - John.
Do you also gave a copy of gemray? This will map the reflections and give you an idea of where the head shaddow, or dark bits will be in the finished stone.
If you have a copy, set the background colour to a colour that contrasts with your stone colour. The default us white so windowing looks like syrface reflections. Changung the colour gives you an idea of how much the stone windows.
Lastly, open up 3 copues of gemray. Leave 1 with the original stone settings, it is the control. Then optimise the other 2 designs using different settings and compare all 3.
If your design is an apex design, or has a small table, set the minimum scaling range below the preset minimum, which is the ratio to achieve the critical angle. Some designs optimise below CA, deflections from other facets become more important, or fill in the gaps.
Hi Gordon, no I have not purchased Gemray, just Gemcad. Gemcad will allow you to shine light rays into a stone and see where they reflect to. I have been using this up to now. Gemray certainly looks good though, I should think about it. Cheers!
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