The hallmark 375 means the jewelry piece is made of 9 karat gold. A hallmark on gold jewelry, such as the hallmark 375, is a symbol used to certify the purity of the gold. The mark also points to the manufacturer, the assay office that certified the gold and the year it was tested.
Many Asian countries such as China, as well as countries across Europe, voluntarily hallmark gold, but the practice of hallmarking is required for all gold pieces sold on the market in the United Kingdom. The United States, on the other hand, does not require this marking, but rather focuses on listing the karat value of a piece of gold, as stated by the World Gold Council.