Mine is transparent rock crystal. I always carry one with me, letting it sunlight as suggested by the C's.
The funny thing is that within a few hours of buying the stone, it broke inside. When I looked at it, I said to myself, "Oh, it broke!" But no, the exterior remains impeccable, it "splinters" inside. It was weird, because I did not fall or take any blow.
Now, in fact I like that detail.