What is a hallmark? 

The word hallmark originates from the fifteenth century when London craftsmen were first required to bring their artefacts to Goldsmiths' Hall for assaying and marking.  The hallmark was probably the first form of consumer protection and is a quality control mark placed on gold, silver and platinum to provide you with a guarantee that what you are buying is of the legal standard. The Hallmark on an article shows it has been tested by an independent Assay Office. In the UK there are four Assay Offices which are legally empowered to test precious metal objects, and to hallmark them. These offices are in London, Sheffield, Birmingham, and Edinburgh.