  Polycrystalline chunk. 
This is basically just ordinary magnesium metal, but it's been broken open to reveal the crystal structure inside.  It is fairly high purity and may have been cooled somewhat slowly to allow the formation of larger than usual crystals, but other than that it's nothing special. 
Source: SoCal (Nevada), Inc 
Contributor: Theodore Gray 
Acquired: 28 April, 2004 
Price: $15 
Size: 4" 
Purity: 99.9%
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