Recommended with reservations, for use in metal clay But, this is what Jane wrote.
I bought some quite awhile ago and some of them turn blue, orange, grey, white, etc. What I did was lay them all out on my firing brick and heat them all too glowing red. I held them there for a couple of minutes. Some of them stay black!! I'm then able to use them in my metal clay creations and they stay black! The other colors aren't bad though. It seems like the smaller the stone the less the color stands up to the heat. When I heated the 5mm stones some of them went a bit clearer but the color shift was much less obvious.