Killas is a type of rock that is composed of sedimentary sea bed material that has been metamorphosed by the enormous amount of heat that was produced when large quantities of granite were formed nearby. It occurs both in Devonian and Carboniferous strata, and is often found near, or in direct association with, valuable metal ores, such as those of copper and tin. The term "Killas" is a name that originated in the mines of southwest England. Around sixty percent of Cornwall lies over Killas, and this factor, combined with its workability, has resulted in it being widely exploited as a building stone.