...is a form of photochromism that occurs in a few minerals; the ones I’m aware of are forms of Sodalite. The piece I show here is Hackmanite, which is probably the best known, and was the first type I learned about.
[[I must note that the first and last photos are identical. I don’t have a photo of the piece in its fully bleached condition because I showed the phenomenon to some friends before it occurred to me to write this, and my “Before” photo from this set (which, in any case, is nearly identical to the photo here) is blurred. Grump.]]
Here’s what a piece of Hackmanite looks like when it is mostly bleached, which is its normal condition:
I illuminated the piece with longwave UV (provided by a 4-Watt BLB fluorescent tube) for about 2 minutes. It doesn’t take that long if I put the bulb right on top of the piece, but I wanted to show you the fluorescence, so I moved the lamp a few cm away.
The fluorescence shifts toward the red during the UV exposure, as the sample darkens. It is actually brighter than it appears here; the camera picked up enough UV to dominate the photo.
Here’s what the piece looks like afterward:
This is a nice deep color change; some samples are not as good, some are even better.
It usually takes a while (days to weeks, IIRC) for the color to fade if you leave the sample in the dark or in sunlight; but it occurred to me that I might be able to speed up the process. Here’s what happens when I shine a rather bright white LED flashlight on the sample for a minute or two:
...And here, again, is what it looks like afterward:
Other tenebrescent minerals can exhibit different colors Tugtupite, for example, at least in the photos I’ve seen, goes to cherry red.