The Metallurgy of Karat

Hold a piece of chemically pure gold and the first surprise is not the colour but the compliance. Twenty-four karat sheet takes a thumbnail impression. A ring made of it will go oval in a pocket. The prongs holding a stone will open under nothing more violent than a coat sleeve. The metal that has carried human wealth for six thousand years is, in its unmixed state, almost useless as a material — and the entire jewellery trade is built on fixing that.

The fix is alloying, and the shorthand for how much fixing has been done is karat. It is worth being exact about the arithmetic, because a great deal of retail confusion lives here. Karat expresses gold content in twenty-fourths by mass. Eighteen karat is 18/24, or 75.0% gold. Fourteen karat is 58.3%. Nine karat is 37.5%. The number tells you nothing about the workmanship, the finish or the durability of the piece; it tells you the mass fraction of element 79 and nothing else.

What the other quarter is doing

In an 18k alloy, a quarter of the mass is not gold, and that quarter is engineered. It sets hardness, colour, melting range, casting behaviour, response to polishing and resistance to tarnish. Change it and you change everything about how the metal behaves on the bench, even though the assay result stays identical.

The three workhorse partners are copper, silver and zinc, with palladium and nickel entering when white is wanted. Copper is the hardener and the reddener. Silver softens the colour towards a greenish yellow and improves the alloy's behaviour under the torch. Zinc is added in small fractions as a deoxidiser during casting, scavenging oxygen that would otherwise leave porosity in the finished piece.

• 18k yellow, a common recipe: 75% gold, roughly 12.5% silver, 12.5% copper — the balanced colour most people picture when they picture gold.
• 18k rose: 75% gold with the balance weighted heavily to copper, typically 20% or more, which is what produces the pink cast.
• 18k palladium white: 75% gold with palladium as the primary bleaching agent, giving a grey-white alloy that is expensive but skin-friendly.
• 9k yellow: 37.5% gold, with the remaining 62.5% dominated by copper and silver — harder, paler, and far more prone to tarnish.

Colour is a solid-state physics problem

Gold is yellow for a reason that most metals are not, and the reason is relativistic. In a heavy atom, the innermost electrons move fast enough that relativistic effects contract their orbitals measurably, which in turn changes the energy gap between the filled 5d band and the partly filled 6s band. In gold that gap falls into the visible range, around 2.4 electronvolts, so blue light is absorbed and the reflected light skews yellow. Silver, one row up, has the same electronic architecture with a wider gap that sits in the ultraviolet — which is why silver reflects the whole visible spectrum and looks white.

Alloying shifts that gap. Adding copper narrows it further and moves the absorption edge, pushing the reflected colour towards red. Adding palladium or nickel disrupts the band structure enough to flatten the absorption across the visible range, producing a metal that reflects more evenly and reads as grey-white. Rose gold is not gold with a dye in it; it is a different electronic material that happens to contain the same mass fraction of gold.

The white gold compromise

No practical gold alloy is genuinely white. Even the best palladium formulations retain a faint warm or grey cast that reads as slightly off against a diamond. The trade's answer, standard since the mid-twentieth century, is to electroplate the finished piece with rhodium — a platinum-group metal that is genuinely white, extremely hard, and applied at thicknesses typically measured in fractions of a micron.

This is rarely explained at the point of sale, and it produces a predictable complaint two years later. A plated layer that thin is a consumable. It wears through first at the underside of a ring shank and at the shoulders, where the alloy beneath shows warm against the still-plated crown. Nothing has gone wrong; the finish has reached the end of its service life and needs redoing, at a cost that is modest but real and recurring.

What the bench actually cares about

Ask a goldsmith about alloys and the conversation will not be about colour for long. It will be about melting range, about how wide the gap is between the temperature at which the alloy starts to melt and the temperature at which it is fully liquid, because a wide gap means a sluggish, mushy casting that traps gas. It will be about whether the alloy work-hardens fast enough to need annealing three times during a single piece. It will be about fire scale, the dark copper-oxide layer that forms under the surface of copper-rich alloys during soldering and only reveals itself halfway through the final polish.

These are the properties that determine whether a design can be executed at all, and they are why workshops develop loyalties to particular alloy suppliers that outsiders find inexplicable. Two 18k yellows with identical assay results can behave like different metals.

Reading the marks

The millesimal number stamped inside a band — 750, 585, 375 — is the same statement as the karat figure, expressed as parts per thousand. It is the modern international convention and it is unambiguous in a way that '18k' printed on an unhallmarked import is not. In jurisdictions with a compulsory hallmarking regime, that number has been verified by an independent assay office rather than asserted by the manufacturer, which is the entire point of the system and the subject of a separate piece in this archive.

For the buyer, the practical hierarchy is short. Purity sets intrinsic value. The alloy design sets how the object survives being owned. And the finish, in the case of white gold, sets how often you will be back at the jeweller's counter. Those three facts are independent of one another, and only the first is printed on the metal.