Rhodium — The Most Reflective of the Platinum-Group Metals
Rhodium — The Most Reflective of the Platinum-Group Metals
A rare, hard, tarnish-resistant white metal used as electroplated finish on white gold, silver, and platinum jewellery
Rhodium is a platinum-group metal — element symbol Rh, atomic number 45 — distinguished in jewellery use by its brilliant white reflectivity, its hardness, and its near-complete resistance to tarnish. It is the most reflective of all metals to visible light, and is applied as a thin electroplated layer over white gold, sterling silver, and platinum to produce the bright, neutral white finish that the contemporary market reads as white. Annual global production of rhodium is under thirty tonnes, the lowest among the platinum-group metals; the metal is correspondingly more expensive than gold or platinum on a per-ounce basis and is essentially never used in solid form in jewellery. The contemporary supply chain runs from primary mining in South Africa and from byproduct streams of nickel-platinum production in Russia, through refining at a small number of specialist precious-metal houses, into the industrial and jewellery markets.
Properties
Rhodium has a Mohs hardness of 6, a melting point of 1964 degrees Celsius, and a density of 12.4 grams per cubic centimetre. It is essentially inert to acid attack — even aqua regia attacks rhodium only slowly — and to atmospheric oxidation at ambient temperature, which is why a rhodium-plated surface stays bright while underlying alloys would tarnish, oxidise, or yellow. The metal is mined principally in South Africa and Russia as a byproduct of platinum and nickel refining, and most industrial supply is consumed in automotive catalytic converters, where its catalytic activity in the conversion of nitrogen oxides supports modern emissions regulations. Jewellery is a small but visible part of the rhodium market, accounting for under five per cent of annual demand.
Optical properties are the other defining feature. Rhodium has a near-flat reflectivity curve across the visible spectrum, returning more than seventy-five per cent of incident light at all visible wavelengths and producing the characteristic bright, neutral white appearance. No other practical jewellery-finishing metal matches this combination of high reflectivity and tonal neutrality.
Use in jewellery
Rhodium is applied by electroplating, with the article serving as the cathode in a bath of rhodium sulphate or rhodium phosphate electrolyte. The plate thickness in jewellery work is typically between 0.1 and 1.0 microns. Below 0.1 microns, the coating is too thin to be wear-resistant; above 1.0 microns, the cost rises sharply without proportionate visual gain. The plate is hard enough to resist most casual wear, but will eventually thin and reveal the warmer tone of the underlying metal at points of contact — the inside of a ring shank, the underside of a bracelet link, the prong tips of a setting.
Re-plating is the standard refresh. A piece sent for rhodium refresh is cleaned, lightly polished to remove worn metal at the contact surfaces, and re-plated; the cycle is typically every twelve to thirty-six months for daily-wear rings and longer for occasional-wear pieces. The economics are favourable: the cost of rhodium re-plating at retail is generally a small fraction of the price of the original piece, and the visual restoration is dramatic.
Market and supply
Rhodium spot pricing is volatile relative to gold and platinum because of the small market and concentrated supply. Prices spiked above twenty-five thousand US dollars per troy ounce in early 2021 before correcting, and have ranged widely since. The volatility affects the cost of re-plating modestly because the actual rhodium content of a plating job is small, but it affects industrial demand and primary supply more substantially. Jewellery buyers see the volatility as a footnote rather than as a structural concern.