Ruthenium — Platinum-Group Hardener and Dark-Finish Plating Metal
Ruthenium — Platinum-Group Hardener and Dark-Finish Plating Metal
The black-grey platinum-group metal used to harden platinum alloys and to plate jewellery in dark tones
Ruthenium is a platinum-group transition metal, atomic number 44, characterised by extreme hardness, brittleness, and a distinctive dark grey to black colour. In jewellery, ruthenium has two principal applications: as a hardening alloy in high-purity platinum, where small percentages improve scratch resistance and durability, and as an electroplated finish that produces a dark grey or black surface on silver, white gold, and base metals. The metal is rarely encountered as a major component of jewellery alloys, but its specialist roles are well-established and increasingly common in contemporary design.
The metal and its properties
Ruthenium has a Mohs hardness of approximately 6.5, making it harder than most precious metals and substantially harder than platinum (4 to 4.5) or gold (2.5 to 3). It is dense (specific gravity 12.4), highly resistant to corrosion and tarnish under normal atmospheric conditions, and chemically inert in the presence of acids and most reagents. The native colour is a dark silvery grey that appears black in thin layers, giving ruthenium plating its characteristic appearance.
The metal is brittle in pure form and cannot be worked into wire, sheet, or castings on its own. Its industrial role is principally as an alloying element in platinum-group metals and as an electroplating substrate. Ruthenium production is small relative to the other platinum-group metals — typically a few dozen tonnes per year worldwide — and the metal is mined principally as a by-product of platinum and palladium operations in South Africa and Russia.
Ruthenium in platinum alloys
Platinum jewellery alloys use ruthenium as a hardening element at concentrations typically between 5 and 10 percent. Pt950Ru and Pt900Ru are the standard formulations: 950 parts platinum to 50 parts ruthenium, or 900 parts platinum to 100 parts ruthenium, with the balance corresponding to the marked fineness. The Pt950Ru standard is the dominant European platinum jewellery alloy and is widely used in fine ring work where the ruthenium contribution improves both scratch resistance and the alloy's ability to hold fine prong and bezel detail under stress.
The competitor alloy is Pt950Ir (5% iridium), which has different working characteristics: iridium-platinum is slightly softer in the worked state but work-hardens more on stress, while ruthenium-platinum is harder in the as-cast state but less ductile. Workshop preference between the two is largely a matter of fabrication method; ruthenium-platinum favours casting work and iridium-platinum favours fabrication and wire-drawing.
Ruthenium plating
Ruthenium electroplating deposits a thin layer of metallic ruthenium onto a base substrate, producing a dark grey to nearly black surface finish. The plating is harder and more wear-resistant than rhodium plating (the standard white finish for white-gold work) and significantly harder than gold or silver plating. Typical thicknesses are 0.1 to 0.5 microns; thicker deposits are technically possible but become brittle and crack-prone.
Ruthenium plating is used principally to achieve the contemporary dark finishes that have become common in men's jewellery, oxidised-look pieces, and as a contrast against bright rhodium or yellow-gold elements. The finish is durable in normal wear but will eventually wear through, particularly on high-friction contact points such as the inside of ring shanks and the underside of bracelets. Reapplication is straightforward and is offered as a service by most fine-jewellery workshops.
Other dark-plating options include black rhodium (a rhodium plating with additives that produces a darker finish, less durable than true ruthenium), oxidised silver (a chemical patina on the silver surface itself, less wear-resistant still), and PVD coatings (physical vapour deposition, producing thin hard ceramic layers in various colours). Ruthenium plating sits in the middle of the durability ladder for dark finishes.
In the trade
Buyers should be aware that black gold in retail descriptions almost always refers to a ruthenium- or black-rhodium-plated white-gold or yellow-gold piece, not to a bulk-coloured alloy; bulk black gold alloys exist but are uncommon in fine jewellery. The ruthenium content of platinum alloys is rarely disclosed at retail but is established by the standard platinum hallmarks — Pt950 with the workshop's mark indicates 950-thousandth platinum with the alloying element conventional to the workshop's standard.