Rhodium

Rhodium was discovered in 1803 by the English chemist William Hyde Wollaston, who found it in the residue left after dissolving crude platinum. He named it from the Greek rhodon, a rose, because a salt of the metal he prepared was a deep rose-red. Rhodium is a rare, hard, silvery-white member of the platinum group, exceptionally reflective and corrosion-resistant, and at times the most valuable of all the precious metals by weight. Rhodium is prized for its brilliant, untarnishing finish, used to plate white-gold jewellery and mirrors, and above all as a catalyst in the converters that clean petrol-engine exhaust, a demand that has periodically driven its price well above that of gold or platinum.

Wollaston discovered rhodium and palladium within months of each other while working through the same batch of crude platinum ore, disclosing palladium first, anonymously and almost playfully through a mineral dealer's shop window, before formally announcing both metals to the Royal Society. Rhodium's price has since proven extraordinarily volatile precisely because its supply is so thin and its demand so concentrated in catalytic converters: in 2008 it briefly traded above ten thousand dollars an ounce, several times the price of gold, only to collapse within months as the global financial crisis cut car sales and industrial demand.

Wollaston named rhodium for the rose-red of its compounds, one of a cluster of elements whose names commemorate colour, alongside chromium the coloured, rhodium the rose, and iridium the rainbow, its sister element found in the same platinum residues by Wollaston's colleague Smithson Tennant. The metal itself is a brilliant white and is prized for the untarnishable rhodium plating on fine jewellery and mirrors, and for its role in the catalytic converters that clean vehicle exhaust. Rhodium has no ancient or esoteric tradition, being a nineteenth-century discovery among the platinum metals; its poetry lies entirely in the rose that named it.

Rhodium's extremely high melting point, over 1960 degrees Celsius, and its resistance to being cast in ordinary moulds meant early chemists and jewellers had to work it as a powder, sintering or electroplating it rather than melting and casting it directly. That electroplating process remains rhodium's most familiar consumer use today: most white gold jewellery sold is actually a pale yellow-tinted alloy finished with a thin rhodium plating to achieve a bright white, mirror finish, a coating that gradually wears thin and needs periodic professional replating to keep the piece looking its brilliant, rose-named metal's true colour.