Cobalt

Saxon miners in the sixteenth century kept hitting an ore that looked like silver, yielded no metal, and gave off choking fumes when roasted — arsenic, as it turned out — so they blamed a malicious mountain sprite, the Kobold, and the name stuck when Georg Brandt isolated the element in 1735. Cobalt blue has coloured glass and glaze since Egypt and Persia, and it is the deep blue of Chinese Ming porcelain. Biologically it sits at the centre of vitamin B12, the only known biological role for the element. Cobalt-60 is the standard high-activity gamma source for sterilising medical supplies and for radiotherapy. Today most cobalt goes into lithium-ion cathodes and superalloy turbine blades, and roughly 70% of world supply comes from the Democratic Republic of the Congo, which makes the supply chain both concentrated and ethically fraught.