Electrowinning plate. 
This is almost certainly a plate that came from the electrolytic extraction of cobalt from its ore, or from an electrolytic purification process.  Cobalt was dissolved in a solution and electricity was used to plate it out onto an electrode.  Very pretty, very heavy. 
Source: eBay seller akilawolf 
Contributor: Theodore Gray 
Acquired: 5 June, 2003 
Price: $15.50 
Size: 1.2" 
Purity: 99.9%
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