What is Silver?

Silver is a highly malleable (flexible / pliable) precious metal with a brilliant white metallic lustre that can take a high degree of polishing. This has led to its huge popularity in jewellery. The fineness of the silver content is expressed in parts per 1000.  Silver is alloyed (mixed) with another metal to make it more durable.  This other metal is usually copper.  

The minimum fineness of silver in Sterling Silver is 925 parts silver, hence the term 925 silver jewellery and it has been known as this since the 13th Century.  The name for the currency of the UK - pound sterling, reflects the fact that it was one troy pound of sterling silver.  A troy pound is 12 ounces.  Troy pounds are now generally only used to measure silver and gold.  A troy ounce is approximately 31 gms.

Britannia Silver is only 800 parts silver and cannot therefore be described as Sterling.
Fine silver is generally 999 parts silver which is naturally much softer and entirely unsuitable for some items of jewellery.  

Gold is a softer precious metal than silver. Gold and jewellery are synonymous, partly because of its use throughout history and may have been the first metal used for ornamentation and ritual. Gold is of course yellow and other colours seen such as red and white are simply alloys of gold. The final colour is dependent on the ratios and type of alloys added. It's cost is of course determined by its rarity but also the appearance and character of the metal itself.
Like silver it is alloyed with another metal to make it more durable and this mixing is named by carat (ct) or KT for the USA, as well as the parts per 1000. A CT (KT) is one twenty-fourth of the total amount of gold contained in the piece. 

9ct gold is 375 parts of gold per 1000
15ct gold is 625 parts of gold per 1000
18ct gold is 750 parts of gold per 1000
22ct gold is 917 parts of gold per 1000
24ct (pure gold) is generally 999 parts of gold

The USA quite often uses
10KT  - 417 parts of gold per 1000
14KT  - 583 parts of gold per 1000
18KT - 750 parts of gold per 1000

By knowing this, any piece of jewellery with either a CT (KT) or a number can be assessed by you for its quality.
Please refer  the Hallmarking Information Article for further information