Platinum vs Gold

Put them side by side in a case and they look like the same idea in two colours. They are not. Gold is bought to be kept and platinum is bought to be used up, and the industries that consume them share almost no customers.

For jewellery the short answer is platinum if you want the setting to outlive you and do not mind the weight, gold if you want the widest choice at the lowest cost per gram of finished object. For everything outside a jeweller's window the two are not alternatives at all.

The ring weighs more, and it is not an illusion

Platinum is denser than gold — one of only a handful of elements that are — and the difference in a finished piece is amplified by how each metal is alloyed.

Gold is too soft to hold a stone at anything near full purity, so rings are made at 18 karat, 75% gold, or lower. Platinum is worked at 95%. Combine the higher intrinsic density with the higher purity and a platinum band of a given design comes out roughly a third heavier than the same design in 18 karat gold, containing considerably more precious metal by weight.

The workshop cost differs too. Platinum melts several hundred degrees higher than gold, which means different torches, different investment materials and slower work, and it work-hardens as it is formed. Some of the price difference at the counter is labour rather than metal.

The behaviour in wear is the part buyers notice years later. When platinum is scratched, the metal displaces rather than flaking away; the surface acquires a soft matte patina but the mass stays. Gold abrades, and prongs on a gold setting thin measurably over decades. Jewellers repair gold settings far more often than platinum ones, which is a maintenance cost that never appears on the original receipt.

Platinum's day job is making other reactions happen

Roughly two-thirds of platinum demand has nothing to do with adornment. Its surface binds reactants just firmly enough to weaken their bonds and just loosely enough to let the products go, which is the whole art of heterogeneous catalysis, and no cheap metal does it as well.

• Autocatalysts. Platinum, palladium and rhodium in a washcoat on a ceramic honeycomb convert carbon monoxide, unburnt hydrocarbons and nitrogen oxides. Platinum dominates in diesel aftertreatment; palladium took over most petrol applications, and the two are substituted back and forth as their prices diverge.
• Nitric acid. Ammonia is oxidised over a woven platinum-rhodium gauze in the first step of the Ostwald process, which is where the world's nitrate fertilisers and nitric acid begin.
• Refining. Catalytic reforming, which raises the octane rating of petrol, runs on platinum-rhenium catalysts.
• Fuel cells and electrolysers. Proton-exchange membrane stacks need platinum at the electrodes, and the amount required per kilowatt is one of the main things hydrogen developers are trying to reduce.

Then there is medicine. Cisplatin, discovered when Barnett Rosenberg noticed in the mid-1960s that platinum electrodes were stopping bacterial cells dividing, became one of the most widely used chemotherapy drugs ever developed, and platinum-based agents remain standard treatment for several cancers. Gold's therapeutic career is thinner and largely historical.

Gold was supposed to be a useless catalyst

For most of the twentieth century gold's chemical indifference was assumed to make it worthless as a catalyst. That turned out to be a statement about bulk gold. In 1987 Masatake Haruta showed that gold divided into particles a few nanometres across oxidises carbon monoxide at temperatures below freezing — an activity bulk gold does not have at any temperature. Nanoparticulate gold catalysis is now a substantial research field, though it has not displaced platinum anywhere industrially.

The metal the kilogram used to be made of

Platinum's combination of a very high melting point and near-total chemical indifference makes it the default material for laboratory measurement. Crucibles for fusing rock samples are platinum. Type S and Type R thermocouples are platinum against platinum-rhodium. The platinum resistance thermometer is the defining interpolating instrument over most of the international temperature scale, which is to say that a large part of what "temperature" officially means is a property of a platinum wire.

Until 2019 the kilogram itself was a cylinder of platinum with ten per cent iridium, cast in 1879 and kept outside Paris. Gold would have been useless for the job — too soft, too easily scratched, and it deforms under its own handling.

One country makes most of the platinum

The supply pictures are not remotely alike, and this is what makes platinum behave like an industrial commodity while gold behaves like money.

Around three-quarters of mined platinum comes from the Bushveld Complex in South Africa, from deep, labour-intensive shafts that are exposed to electricity supply and to industrial relations in a single country. Russia's Norilsk operations supply most of the rest. A disruption in either place moves the market immediately.

Gold production is spread across dozens of countries with no single producer close to dominant, and the more important number is the stock rather than the flow: very nearly all the gold ever mined still exists, somewhere, in a form that can be melted. Annual mine output is a small percentage of that standing pile, so gold's price is set by what holders will do rather than by what miners produce. Platinum has no comparable above-ground inventory; secondary supply comes mainly from recycling scrapped catalytic converters, which is also why converter theft became a recognisable category of crime.

The price inversion

Platinum traded at a premium to gold for most of the decades up to the 2008 financial crisis, and has traded below it for most of the years since. Two things drove the switch. Diesel's share of the European car market collapsed after 2015, taking a large slice of platinum's biggest end-market with it, while gold attracted sustained monetary demand from central banks and investors.

Anyone quoting a fixed ratio between the two is quoting a snapshot. The structural point is that platinum's price answers to factories and gold's answers to balance sheets, so they are not even responding to the same news.

Wearing it, or using it up

• A setting that must hold a stone for fifty years — platinum.
• The most finished jewellery per unit of budget — gold, at 14 or 18 karat.
• Skin that reacts to nickel — platinum, or a palladium-alloyed white gold rather than a nickel one.
• A crucible, a thermocouple or a resistance thermometer — platinum.
• Any catalytic duty at all — platinum, and gold only in the laboratory.
• A holding meant to be indifferent to the industrial cycle — gold.