"An 18ct solid gold necklace"

A golden laurel crown, thought to date from 3rd/4th century BC Cyprus
Public DomainA golden laurel crown, thought to date from 3rd/4th century BC Cyprus - Credit: Andreas Praefcke
 Carat (or karat, abbreviated to 'ct' or 'K') is a system used to indicate the purity of gold. It is measured as 24 times the purity divided by mass, which means that 24 carat is the purest form (the object will be at least 99% pure gold). 18 carat gold is 18 parts gold, six parts composed of another metal.

The carat value is thought to have been established in AD 309 by Roman Emperor Constantine I.

Different carat marks are more common in different countries – for example, the gold produced in the UK is commonly 9 – 22 carat, whereas 24 carat is associated with the Middle East, China, Malaysia and Indonesia. 18 carat gold is the standard gold value found in the southern Mediterranean countries such as Italy.