A hallmark is a set of small stamps certifying the metal content of a piece. What those stamps prove depends heavily on the system that applied them.
The mark records an independent test
In a full hallmarking system, an office independent of the maker tests a sample from each item and applies marks confirming the fineness found.
Because the tester has no interest in the outcome, the mark carries weight that a manufacturer's own stamp does not.
The marks typically identify the fineness, the office that tested the piece and the sponsor who submitted it, and often the year.
Compulsory and voluntary systems differ in coverage
Some countries require independent assay before precious metal goods can be sold above a small weight threshold, so almost everything on sale is marked.
Others rely on the maker's own declaration backed by trading standards enforcement, which places the burden on the buyer to trust the seller.
An unmarked piece in the second kind of market is unremarkable, while in the first it suggests the item was not sold through normal retail channels.
Fineness standards are not universal
Recognised standards vary between jurisdictions, so an alloy that is legal to sell as a given description in one country may not qualify in another.
Tolerances also differ, with some systems permitting a small negative deviation and others requiring the stated fineness as a minimum.
Imported goods must therefore sometimes be re-tested and re-marked before they can be sold, adding cost that reaches the retail price.
Mutual recognition reduces duplication
International conventions allow a mark applied by one participating country's assay office to be accepted by others without retesting.
This works because the participating offices operate to a common standard and inspect each other's procedures.
Coverage is partial, so trade with countries outside such arrangements still involves testing at each border where marking is compulsory.
What a mark does and does not guarantee
A genuine hallmark certifies the fineness of the metal at the point tested. It says nothing about gemstones, workmanship or the value of the finished item.
Marks can also be forged, which is why dealers test rather than read on high value purchases, and why the mark supports a test rather than replacing it.
For a buyer, the practical value of the system is that it makes misdescription an offence with an evidential trail, rather than a matter of dispute between two parties.