Real Dictionary

hallmark
The hallmark of something or someone is their most typical quality or feature.
countable noun: A hallmark is an official mark put on things made of gold, silver, or platinum that indicates the quality of the metal, where the object was made, and who made it.

hallmark in American English
an official mark stamped on British gold and silver articles, orig. at Goldsmiths' Hall in London, as a guarantee of genuineness
to put a hallmark on
noun: an official mark or stamp indicating a standard of purity, used in marking gold and silver articles assayed by the Goldsmiths' Company of London; plate mark
transitive verb: to stamp or imprint (something) with a hallmark

hallmark in British English
noun: an official series of marks, instituted by statute in 1300, and subsequently modified, stamped by the Guild of Goldsmiths at one of its assay offices on gold, silver, or platinum (since 1975) articles to guarantee purity, date of manufacture, etc
verb: to stamp with or as if with a hallmark