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Fordite, also known as Detroit agate or Motor Agate, is old automobile paint which has hardened sufficiently to be cut and polished. It was formed from the buildup of layers of enamel paint slag on tracks and skids on which cars were hand spray-painted (a now automated process), which have been baked numerous times. In recent times the material has been recycled into "eco-friendly" jewelry.
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Source of the article : Wikipedia