Sputtering target. 
This is supposedly a boron doped silicon sputtering target.  I have not researched whether that is a sensible claim or not, though it does look and feel like silicon. 
Source: eBay seller matky 
Contributor: Theodore Gray 
Acquired: 10 September, 2004 
Price: $37 
Size: 4" 
Purity: 99%
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