  Mischmetal block. 
A large block of "mischmetal", a mixture of lanthanum, cerium, and a few other random rare earths.  Blocks like this are used for spark effects in movies.  It is incredibly sparky stuff, here I'm holding it up to a small grinding wheel.  The "Spin" and "Rotate" videos show this in action. 
Source: eBay 
Contributor: Theodore Gray 
Acquired: 16 April, 2009 
Text Updated: 16 April, 2009 
Price: $30 
Size: 3" 
Purity: <20%
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