an official mark stamped on gold and silver articles in England to attest their purity

Centuries ago, King Edward I of England decreed that gold and silver had to be tested and approved by master craftsmen before being sold. Later, London artisans were required to bring finished metal goods to Goldsmith's Hall to be checked, and if those items met the quality standards of the craft-masters there, they would be marked with a special stamp of approval. (The process is much the same today.) At first, people used hallmark to name that mark of excellence from Goldsmith's Hall, but over the years the word came to refer to any mark guaranteeing purity or genuineness, and eventually to name any sign of outstanding talent, creativity, or excellence.