Fordite aka Detroit Agate Cabochon
~ 22 x 13 x 5 mm
Fordite, also known as Detroit agate or Motor City agate, is old automotive paint which has hardened sufficiently to be cut and polished. It was formed from the buildup of layers of enamel paint slag on tracks and skids on which cars were hand spray-painted, which have been baked numerous times. (from Wikipedia)