Uranium Thorite
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This is a very rare thorium mineral. Not much to look at, but it has a well-defined crystal structure and it's hot enough that when it fell under a bunch of stuff, I had no trouble locating it with a Geiger counter. (This is one of the great advantages of radioactive things: You can never really loose them. Not so the osmium pellet I'm probably never going to find.)
The price reflects the rarity of this species.
Source: eBay seller mineralman999
Contributor: Theodore Gray
Acquired: 3 June, 2005
Price: $90
Size: 0.5"
Composition: (Th,U)SiO