The hallmark is a set of component marks applied to precious metals.  Collectively they show that the item has been independently tested and guarantees that it meets the legal standard of purity.

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​​Here in the UK,  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.)

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As well as providing peace of mind to consumers, the hallmark gives provenance to jewellery, stating not only what metal the item is made from, but where it was hallmarked and by who it was submitted.