“Fordite,” also known as “Detroit agate” or “Motor City agate” is not rock, but rather, the buildup of paint overspray baked over time on the walls and assembly stands in manufacturing plants. When not sold as solid chunks, it can turned into pendants, charms, cufflinks, and keepsakes.

https://www.reddit.com/r/sportsandclassiccars/comments/1suuczo/fordite_also_known_as_detroit_agate_or_motor_city/