These gorgeously striated pieces are called Fordite. They were created when, back when cars were spray painted by hand, the run-off would collect in tracks and other depressions in the shop floor. High heat used to seal the paint also hardened the run-off, layer by layer, until Fordite resulted.
As the car-painting process changed, the existing crop of Fordite became the only crop. The assumption is that workers saw the layers and brought the stones out of the old factories. Polished up, the by-product of car manufacturing is both pretty and a preserved history of paint colors and compositions.
More at My Modern Met
http://www.mymodernmet.com/profiles/blogs/fordite-stones
