Aluminium

Aluminium is 8% of the Earth's crust and yet was a precious metal until 1886, because it is bound so tightly to oxygen that no ordinary smelting reduces it. Napoleon III reputedly served his most honoured guests on aluminium plates while everyone else made do with gold, and the Washington Monument was capped in 1884 with a 100-ounce aluminium pyramid that cost about the same as a skilled worker's annual wage. Then in 1886 Charles Hall in Ohio and Paul Héroult in France, both twenty-two and working independently, hit on dissolving alumina in molten cryolite and electrolysing it; the price collapsed by a factor of two hundred within a decade. The metal survives in air because it instantly grows a 4 nm oxide film that seals it, and recycling it takes about 5% of the energy of smelting new metal.