Karat and fineness describe how much gold is in an alloy. Learn how those marks differ from plating, why alloys change colour and what a hallmark can confirm in a specific market.
Gold in a piece of jewellery
Gold is a metal, but a gold-coloured jewel is not necessarily made from one uniform material. Pure gold is very soft for many jewellery uses, so makers often combine it with other metals to form an alloy. An alloy still contains gold throughout its metal body; a plated object has gold only at the surface. The distinction begins with the material, not its colour.1,2,3
Two ways to state the gold content
Karat divides an alloy into 24 parts by weight: 18-karat gold has 18 parts gold and six parts other metal. Millesimal fineness states the gold share in parts per thousand, so 18-karat gold corresponds to 750 fineness. Marks and acceptable standards vary with the market: for example, UK guidance gives 375 as the fineness mark for 9-carat gold, while GIA notes that 14-karat pieces may be marked 583 or 585. Read a number in the context of its full mark and market.1,2,4
The alloy shapes colour and character
The karat number states the share of gold, not a fixed recipe for the remaining metal. Silver, copper and other alloying metals can alter colour and physical properties, so yellow, white and rose gold describe families of alloys rather than one universal formula. Some white-gold jewellery also has a rhodium surface coating; the underlying alloy and any coating are separate parts of its construction.2,3
