Copper Petrified Wood.  
The tag that came with sample reads as follows:Petrified wood from the Nacimiento Mine 
Replacement minerals could include Malachite, Chalcocite, Azurite, Cuprite, Covellite, Digenite, Bornite, Plancheite, Iron Oxide, Pseudomalachite, Pyrite, Cubanite, Brochanite, Antlerite, Elemental Copper, Stromeyerite. But it's the copper we're interested in here!  The green color indicates that copper is a major constituent of whichever minerals make up this sample.  In fact, I traded this sample for a few of my strange copper nodules. 
Source: Calvin Webb 
Contributor: Calvin Webb 
Acquired: 1 September, 2003 
Price: Donated 
Size: 2" 
Composition: Cu
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