There are four primary precious metals - gold, silver, platinum and palladium. The most expensive metal is Rhodium, a silvery coloured metal used principally for its reflective properties.
Gold for jewellery use comes in 5 main types or fineness, determined by the proportion of pure gold.
9 carat - 37.5% gold
14 carat - 56% gold
18 carat - 75% gold
22 carat - 91.7% gold
24 carat or pure 100% gold.
In addition jewellers use gold plating, which is a base metal coated in gold via electrolysis. Another form is gold fill a term used to describe an amalgam of gold mixed with brass or bronze. This is used by jewellers as an economical substitue for gold as it is highly durable, the colour will not fade/wear and costs far less than gold on its own.
Silver comes in two principal forms, silver plate and sterling silver.
Silver plate, like gold plate, is formed by dipping a base metal into a solution for electrolysis, the resultt being a thin layer of silver.
Solid silver comes in three forms, sterling (92.5% pure), britannia (95.8% pure) and fine, or pure, silver. Since silver is a relatively soft metal jewellers tend to use only sterling silver, often described as 925 silver.