I have been a rock/mineral/agate collector since childhood, but had never heard of "fordite" until encountering the word this morning in a J-Walk comment thread, where it was the wrong answer to a question. Fordite is an enamel created when paint accumulates and becomes hardened. The name presumably derives from its being harvested from old Ford automobile manufacturing plants.
The original layered automotive paint slag "rough" was made incidentally, years ago, by the now extinct practice of hand spray-painting multiples of production cars in big automotive factories.The oversprayed paint in the painting bays gradually built up on the tracks and skids that the car frames were painted on. Over time, many colorful layers built up there. These layers were hardened repeatedly in the ovens that the car bodies went into to cure the paint. Some of these deeper layers were even baked 100 times. Eventually, the paint build-up would become obstructing, or too thick and heavy, and had to be removed.Text and photo credit to Fordites.com, where there is a gallery of cabochons.
As the story goes, some crafty workers with an eye for beauty realized that this unique byproduct was worth salvaging. It was super-cured, patterned like psychedelic agate, and could be cut and polished with relative ease!
Sadly, the techniques that produced this great rough years ago, are no longer in practice. Cars are now painted by way of an electrostatic process that essentially magnetizes the enamels to the car bodies. This leaves little, or no overspray. The old factory methods that created this incredible material are long gone.
Addendum: Reposted from 2009 to add a link to some impressive photos of fordite posted at Bored Panda and at My Modern Met.