via here and here.

Fordite, also known as Motor Agate or Detroit Agate are “stones” that were created from the process of spray painting multiple cars at once back in Detroit’s hayday. Paint would drip and collect on the tracks and skids that the cars were moved through, and baked many times. After many years, so much paint would build up that it was removed and taken home by workers as gifts for their wives and children. The stones could be cut and polished easily and these days are being used to make rather hideous jewelry. You can still get stones or the original slabs on ebay. Might be a piece of history worth having!

-Claudia Brown