Precious metals are rarely used in their purest form but are usually alloyed with other metals for workability, durability, wearability. It isn’t possible to detect an article’s precious metal content by sight or touch. Therefore, it is a legal requirement for an Assay Office to hallmark articles containing precious metals if they are described as such. (Weight exemptions apply for items below certain weights. For silver 7.78g, gold and palladium over 1g, and platinum over 0.5g to have a UK recognised hallmark.)
To comply with the UK Hallmarking Act (1973), all precious metals above a specified weight must be hallmarked in order to be sold to the public.
I am registered with The Goldsmiths’ Company Assay Office in London; all my items above the required weights are hallmarked. For example all my sterling silver items weighing 7.78g or over are hallmarked.
Items have to be sent for hallmarking, this can incur a delay in custom orders, please consider this if items are required for a specific date. All items in my Etsy shop which are available for immediate dispatch will have already been hallmarked.
Generally, other items selling for £50 and over are also hallmarked, this is not a legal requirement but provides providence when buying more expensive jewellery.
(Information source and for more information visit https://www.assayofficelondon.co.uk)