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Further thoughts: In 1958, when I was a shiftboss at the underground Mosaboni copper mine in India, a Mine Captain, Jimmy Evans, had previously worked single-handedly in a former lead mine in Derbyshire, working it for the fluorspar left behind by the lead miners. To them it had been of no commercial value (= "gangue"), whereas in the 1950's there was a great market for it. Equally, lead miners in the middle ages threw away sphalerite, zinc sulphide, as being useless (and thus to them, gangue). If you put galena, lead sulphide, into a furnace you get a nice puddle of lead in the bottom of the furnace. Throw sphalerite into the same furnace, and you get a lot of white smoke (vaporised zinc oxide) but no metal. The name "sphalerite" was concocted by some early chemist from the Greek for "deceiver". Production of metallic zinc was developed in China and in India near Zawar, Rajasthan, in the 1100s AD, and imported into Europe as "Indian tin" whereas the first time it was produced in England was in the mid-1700s near Bristol - perhaps some ship's captain had come back with a tale of what he had heard about its production in India, and passed on the idea?? (The Romans and their successors produced brass, but they did so by heating powdered copper metal with calamine). Tony Brewis