I have a group of small amethyst and recently I have been trying to learn identification with my instruments. Curious, I looked at them with my uv light. What happened was strange. Some turned clear, two turned dark blue and two turned pale purple. And , after several hours they returned to the color they were, purple amethyst. Anyone have an idea what is going on? One stayed dark blue the rest returned to original color.
Tamara,
Fluorite vs. Amethyst? Also, Hackmanite will change color under UV and retain the darker purple for a few minutes up to a few hours, before turning back to a light pink/lavender.
If I remember Amethyst does fluoresce under UV. What instruments do you have available?
https://www.mindat.org/min-1789.html Hackmanite. You can also view information on the Amethyst and Fluorite at this site if the other links are not available.
I would do a Specific Gravity test on each one separately.
Fluorite Specific Gravity: 3.180; massive material with impurities 3.0-3.25.
Amethyst Specific Gravity: 2.651
This would help identify one group from another. I would also keep tabs on which one’s colors changed over time. That is a unique trait.
-Troy
UV light is only good for fluoresence… you did not seem to indicate that the specimens seem to “glow” in these different colors. UV light is not a good way to look at ordinary “color” as the color of the gems may change significantly under the UV light especially when there is no “fluorescence” as it can change simply with the thickness of the mineral. Assuming you are talking about the color in ordinary room light and are comparing color prior to UV exposure and post UV exposure … then a true color change resulting from post exposure to UV light is something I have never heard of for amethyst… and I would be inclined to think you are looking at a different blue mineral.
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Thanks. They didn’t turn darker purple. Most lost almost if not all their color, most of which returned several hours later. 3 turned dark blue with no purple and stayed that way. I appreciate the SG idea.
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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
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An update: I was very tired last night when I replied and didn’t fully articulate my response. I forgot to mention that amethyst will loose its purple color permanently with long exposure to sunlight (strong UV) and/or heat. I do not believe it can recover back to its original purple. That is the classic form of photochromism. It is also how a majority of citrine is produced.
Yep… I need more sleep… and less coffee! 
-Troy