Sterling silver is an alloy of 92.5% by mass of pure silver and 7.5% of other metal, usually copper, hence the mark 925.
By law, an item claiming to be 'sterling silver' and weighing over 7.78 grams must be hallmarked.
Before applying a hallmark an assay office will test an item to make sure it is sterling silver by taking a miniscule sample.
I request the full traditional UK hallmarks be applied, as shown, plus my maker's mark. This final mark must be the maker's initials. I am registered with the Goldsmith's' Company Assay Office in London, identified by the panther's head mark.