fordite

Fordite, also known as Detroit agate or Motor City agate, is old vehicle paint which has hardened sufficiently to be cut and polished.
It was formed from the buildup of layers of paint slag on tracks and skids on which cars were hand spray-painted, which have been baked numerous times sometimes up to 100 times. Fordite is no longer produced due to car painting being done by a different process now days.
This ultimately means while stock is difficult to find but still can be. The ultimate stock is limited and will one day end.

So when it's gone it's gone.