What is a hallmark on gold?

A hallmark is an official stamp that shows a piece's purity has been tested.

Rules differ by country, and many countries have their own testing system. A common mark is a number showing purity in parts per thousand, so 916 means 91.6% gold, or 22K, and 750 means 18K. A hallmark helps you check that you get what you paid for, but it does not tell you whether the price is fair.