Detroit agate (Fordite) is one of the by-products of old style car production. Fordite was formed from layers of paint over several tens of years. Old car paint would drip onto the metal racks that transported cars through the paint shop and into the oven. The paint was hardened to a rock-like state thanks to high heats from the baking process. The colors can be used to judge how old they are as car paint was subject to different trends. In the 1940s cars were mostly black or brown enamel while the 1960s ushered in an age of colorful lacquers.