A more thorough explanation:

Definition: A hallmark is an official stamp that is put on articles made of gold or silver by goldsmiths and silversmiths to show that they are genuine. It is also used to refer to a mark of genuineness.

Examples:

• A gold necklace that has a hallmark on it shows that it is made of real gold.
• A silver ring that has a hallmark on it shows that it is made of real silver.
• A certificate of authenticity that comes with a piece of artwork can be considered a hallmark of its genuineness.

The examples illustrate how a hallmark is used to show that something is genuine. In the case of the gold and silver jewelry, the hallmark is a stamp that is put on the piece by the goldsmith or silversmith to show that it is made of real gold or silver. In the case of the artwork, the certificate of authenticity serves as a hallmark to show that the artwork is genuine and not a fake.