925 cn with a d inside square is a hallmark indicating sterling silver that has been assayed and stamped by a government approved assay office.

The number '925' refers to the purity of the silver, which is 92.5% silver and 7.5% alloy. The 'cn' is an abbreviation for 'centigram', indicating that the item weighs 9.25 grams.

The 'D' inside a square symbol is the assay office mark, which indicates the country or region where the item was assayed and hallmarked. It may refer to an assay office in London.