A hallmark is an official quality control mark placed on gold, silver and platinum and provides a safeguard to the consumer.
What is now recognised as a modern hallmark began in the year 1300 when Edward the 1st attempted to regulate the purity of craftsmen's items. An external group called "The Guardians of the Craft" were a group who went from shop to shop to assay the metals used by craftsmen and strike the "Leopard's Head" mark where necessary.
In modern times, hallmarking (where required due to weight) is done by an assay office. There are 4 assay offices in the UK: London, Birmingham, Sheffield or Edinburgh. The metals are tested for purity and fineness and then the correct mark is struck. The mark can be punched onto the metal with a small metal stamp or can now be applied by a laser.