official stamp affixed by guilds or assay offices to gold and silver objects

A hallmark is an official mark or series of marks struck on items made of metal. The purpose is to certify the content of metals such as platinum, gold, silver and in some countries, palladium.

In a more general sense, the term 'hallmark' can also be used to refer to any characteristic which signifies (signals) quality. So, in the 19th/20th centuries, marking steel cutlery as "made in Sheffield" was a guarantee of its quality. Historically, hallmarks were applied by a trusted party: the "guardians of the craft" or, more recently, by an assay office. Hallmarks are a guarantee of certain purity or fineness of the metal, as determined by official metal (assay) testing.