Sterling silver is an alloy of 92.5% by mass of pure silver and 7.5% of other metals, usually copper.

By Law, an item claiming to be 'Sterling silver' and weighing over 7.78gms must be hallmarked. 

Before applying a hallmark they will test an item to make sure it is Sterling silver by taking a miniscule sample.  I request the full traditional UK hallmark be applied, as shown below, plus mymaker's mark.  This final mark must be the maker's initials.  I am registered with the London Assay Office.