A hallmark is a set of marks applied to an article made from precious metals to confirm that it has been independently tested by an Assay Office. The hallmark guarantees that the article conforms to all British legal standards of purity (fineness), guarantees a provenance by telling you where the piece was hallmarked, what it is made from, when it was hallmarked and who sent the article to be hallmarked through the makers mark.
My makers mark is a square containing 'HB' and I am registered with the London Assay office.
Hallmarking is a legal requirement for items being sold over a certain weight (see the bottom of the chart below for this information), but it is optional for items being sold under the exemption weights. It is quite a pricey process and so not all my exemption weight items are hallmarked to keep the cost to you down. However, I can always send any unmarked items to be hallmarked if you would prefer, at cost.