A hallmark is the tiny series of stamps (or engravings) on an item made from precious metal that shows it has been tested by an independent assessor to meet minimum quality and quantity requirements set under law.
In the UK there has been a legal requirement for items to be assayed by a ‘Guardian of the Craft’ since 1300 when Edward I passed the hallmarking statute. This established the basic standard of silver at the sterling standard (92.5% pure silver) and gold at the ‘touch of Paris’ (19.2 carats). Over the years these laws have been amended slightly to add or remove some additional marks, and to tweak the minimum/threshold weights for items requiring a hallmark.
For a more detailed history on the history and evolution of British Hallmarks head over to the Goldsmith’s Company website.