Guide to precious metals

Reading a jewel begins with reading its metal

A stone is rarely alone: it is set, mounted, worn. Understanding the precious metal that holds it is therefore inseparable from gemmology — and it is often there that much of an antique jewel's value lies.

Gold: a matter of proportions

Pure gold — 24 karat, or 999 parts per thousand — is too soft to wear: it deforms and scratches. So it is alloyed. In France the jewellery standard is 18-karat gold (750 parts per thousand of gold, or 75%), marked by the eagle's-head hallmark. 14 karat (585‰) and 9 karat (375‰) are more common in the English-speaking world.

The colours of gold depend only on the alloy, never on the gold itself: yellow gold is alloyed with silver and copper, rose gold takes more copper, and white gold is alloyed with palladium or nickel, then usually coated with rhodium for its cool brightness. Worth knowing: that rhodium plating wears with time and the piece yellows slightly — replating is done by a jeweller for a modest sum. It is not a defect in the jewel.

Silver and platinum

Sterling silver 925 (92.5% silver, alloyed with copper) is the world standard. It tarnishes naturally through sulphidation in contact with air — and that tarnishing, far from being a fault, is even a sign of authenticity: stainless steel does not tarnish. A simple polish restores it.

Platinum 950 is rarer and denser than gold, naturally white — so no rhodium plating to redo — never tarnishes, and is hypoallergenic. It is the metal of the great houses, and its weight in the hand gives it away instantly.

Vermeil or plated: never confuse them

Vermeil is solid silver covered with at least 5 microns of gold: it is a genuinely precious jewel, and it carries a silver hallmark. “Gold plated” is merely a thin layer of gold deposited on brass or copper: it wears through, revealing the base metal, and never carries a precious-metal hallmark. The price difference is enormous, the difference in wording is slight — and that is exactly where nasty surprises hide.