  Torbernite.  
Torbernite is a lovely, lovely green color (I would guess from the copper).  It's also quite radioactive, from the uranium content, and even more so from the mixture of uranium decay products that have built up in it over millions of years. 
Source: eBay seller migalf1 
Contributor: Theodore Gray 
Acquired: 3 June, 2005 
Price: $27 
Size: 1.5" 
Composition: Cu(UO2)2(PO4)2.8-12H2O
 |  
  |  
 |  
  |