Hallmarking is regarded as protection for jewellery buyers and suppliers because it ensures that jewellery items are genuine.
The four metals that are required to be hallmarked are gold, silver, platinum, and palladium.
Items that weigh less than 1 gram in gold or palladium, less than 0.5gm in platinum, or less than 7.78gm in silver are excluded from hallmarking requirements
See below for images of hallmarking.
On a piece of jewellery you will also see the Assay office mark:
Birmingham is represented by an anchor
London is represented by a leopard
Sheffield is represented by a Tudor rose
Edinburgh is represented by a castle