In Britain, Hallmarking has a long history, dating
back 700 years and representing the earliest form
of consumer protection.
A Hallmark means that the article has been
independently tested by an Assay Office and guarantees
that it conforms to a specified legal standard of purity.
This is called the fineness of the metal and the precious
metal content is expressed as a millesimal number.
For example, the number 999 means that the metal is
99.9 percent pure.
Unless specifically exempted, all gold, silver and
platinum articles offered for sale must be Hallmarked.
The Hallmarking Act permits other European
Economic Area Hallmarks and standards of fineness.

Source: www.theassayoffice.co.uk