What are hallmarks and hallmarking?
Precious metals are generally alloyed (mixed) with other metals to increase their durability and are rarely used in their purest form.  It's therefore impossible to detect how much precious metal is contained in an object by sight or touch, so it's a legal requirement to hallmark precious metal if it is described as such. *

The UK Hallmarking Act (1973) governs the requirements in the UK today, but it dates back to the 1300's, when Edward I instituted the assaying (testing) and marking of precious metals. The original aim of Edward I remains unchanged; the protection of the public against fraud and for the trader against unfair competition. Hallmarking is one of the oldest forms of consumer protection. 

What does a hallmark in the UK look like?
A hallmark is made up of 3 compulsary symbols

There may however be additional voluntary symbols for example (a) date letter and (b) the fineness symbol as well as the fineness mark.

There are now 4 Assay Offices in the UK at where articles are tested and marked.

If you refer to the Silver and Gold Information Article you will see the various Metal fineness (purity) marks attributed to some silver and gold.   For Instance I use:

The Assay Office will often mark the least quality of metal purity after testing.  So, for instance an item with silver and a gold set gemstone will receive a 925 silver mark as it not possible to hallmark the gemsetting.  If an item were to have a combination of silver and brass for instance, it will not be hallmarked or will have '+ METAL' stamped if it is clearly distinguishable. The hallmark can only be struck on the precious metal part of the object.

In the image above, there are 5 hallmarks.  This is an enlargement of a hallmarked item of my jewellery. The largest hallmark in this selection is in reality only 2mm x 3mm in size. The Maker's Mark is purchased from the London Assay office for a renewable period of 10 years and as you can see, my initials are contained in a shield. The Maker's Mark cannot be used by anyone else.  I include the fineness symbol as well as the fineness mark on items which I request to be hallmarked.  Any item on this website, if it bears the hallmark will have this picture indicating that it has been sent to the London Assay Office for testing and marking.  If you have any questions as to whether a piece is hallmarked please contact me.

What needs to be hallmarked?
* As with any law, the 1973 Hallmarking Act being no exception, there are exemptions to the hallmarking requirements.
The most applicable for this website being Gold of a weight less than 1 gram and Silver of a weight less than 7.78gm, (based on the precious metal weight only) do not require hallmarking when made in the UK.  Also an article which is intended for dispatch to a destination outside the UK.

The hallmarking of silver jewellery is commonplace throughout the world, and although some major countries – such as the US – don't have a compulsory system, many European countries, including the UK, follow procedures laid out in by the Vienna Convention, with easily comparable hallmarks.  There are 20 Members of the Vienna Convention and 4 Observers - ie they have applied for membership and are in the process of ratifying the Convention until they become full Members.
The Members of the Hallmarking Convention are currently Austria, Croatia, Cyprus, Czech Republic, Denmark, Finland, Hungary, Ireland, Israel, Latvia, Lithuania, Netherlands, Norway, Poland, Portugal, Slovak Republic, Slovenia, Sweden, Switzerland, United Kingdom.  The Observers are currently Italy, Servia, Sri Lanka, Ukraine.

General
In 2010 many silversmiths/goldsmiths in the UK petitioned the government to keep the Hallmarking System as it was being considered for abolishment. Given that it's the most likely the oldest form of consumer protection, this is something that I participated in. The voice of the people won the battle to keep our historic and (I believe), vital hallmarking system.

By far, the most common mark used on sterling silver jewellery in the USA is the metal stamping with the numbers 925.  Jewellery imported into this country will often just bear the 925 Sterling Silver Mark.  It is also important to be aware that some silver jewellery may not be sterling silver, but Britannia quality silver (800 parts per 1000). The  names of the fineness of silver are such because the origins of the hallmarking system lie with Great Britain, with the London Assay Office (operated by the Goldsmiths' Company) being considered the oldest in the world.  

Stamped simply with 925, as with many items under 7.78 grams when imported, may simply mean that the item is exempt from requiring a hallmark from a UK or European Assay office. There are very high costs involved in obtaining marks from an Assay Office, this being one of the reasons why English hallmarked silver has a higher price in general.  Traditionally Tibetan and Bali Silver had a fine content of between 925 - 960. However now it is entirely possible to purchase Tibetan and Bali silver with no silver content at all!