Tamera,
That is very interesting. It definitely puts these stones in the “unknown” category at the moment. Do you know what wavelength your UV light source is? Long-wave (350 - 380 nm), short-wave (250 nm), or somewhere in-between? Here’s a quick reference: Ultraviolet Spectrum - The Fluorescent Mineral Society
I would not expose the stones to UV from this point forward…
@Dion has a good point, under the UV, the material should fluoresce a different color, not change or loose its color hue. I was recently in another topic and provided the following terminology in that conversation:
You observed both of these phenomena. The stone(s) that permanently changed color was most-likely photochromism. And the others that became transparent and then returned to their natural state was tenebrescence. There are not many gem materials and minerals that display these. Amethyst should do neither.
Do you have a Chelsea filter, dichroscope, refractometer, and polariscope? It would also be good to have a field spectroscope. We talked about the SG testing. Have you tried that, yet?
Also, do you have any images of the stones? Thanks!
-Troy