hallmark

Concise Oxford English Dictionary © 2008 Oxford University Press:
hallmark/ˈhɔːlmɑːk/

▶noun

• 1 a mark stamped on articles of gold, silver, or platinum by the British assay offices, certifying purity.

• 2 a distinctive feature, especially of excellence.

▶verb

• 1 stamp with a hallmark.

• 2 designate as distinctive.

– origin C18: from Goldsmiths' Hall in London, where articles were tested and stamped.