Chromium earns its name: chrome yellow, chrome green and chrome oxide were among the strong pigments that transformed nineteenth-century painting and industry, and the metal's ions lie behind some of the most prized colours in the mineral world. The red of ruby and the green of emerald both come from traces of chromium in otherwise colourless crystals, corundum and beryl respectively, so the same element paints two of the classic gemstones in opposite hues. Chromium has no ancient identity, being an eighteenth-century discovery, but its very name makes it, more than any other element, the metal of colour, a fitting associate for the long human fascination with pigment and gem.
Chromium's practical importance rests on the same reactivity that gives it colour: added to steel above roughly ten and a half per cent, it forms an invisible, self-healing oxide layer that resists rust, the principle behind stainless steel, credited to the Sheffield metallurgist Harry Brearley, who identified the alloy's corrosion resistance in 1913 while testing gun-barrel steels. Chromium compounds have a darker industrial history too: hexavalent chromium, used for decades in electroplating and leather tanning, is a recognised carcinogen and groundwater contaminant, the pollution at the centre of the Hinkley, California case later dramatised in the film Erin Brockovich.