  Silicated Magnesium.
This was described on eBay as a heavily silicated magnesium nodule from the Clear Creak mining district in San Benito, California. It sure doesn't look like magnesium metal, but I'm not clear what mineral it actually is.
Source: eBay seller zephyr4
Contributor: Theodore Gray
Acquired: 13 February, 2007
Text Updated: 14 February, 2007
Price: $25
Size: 3"
Composition: MgSi
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