  Manganese oxide pigment tile. 
Manganese oxide has been found used as a pigment in cave paintings from as long as 17,000 years ago.  This tile is not quite that old, but according to the seller it dates to about 1800, making it a good 200 years old. 
Source: eBay seller chrisbuckman 
Contributor: Theodore Gray 
Acquired: 11 March, 2009 
Text Updated: 12 March, 2009 
Price: $45 
Size: 4" 
Purity: <5%
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