hallmark: [18] The hallmark on a piece of gold, silver or platinum, which allows it to be legally sold, is so called simply because it was originally applied in Goldsmiths’ Hall, the headquarters of the Goldsmiths’ Company (a City livery company), where the London assay office was situated. The building is in Foster Lane, to the northeast of St Paul’s Cathedral in the City of London. [hallmark etymology, hallmark origin, 英语词源]