Types of Metals

Sterling Silver: A mixture of pure silver and some other metal, usually copper. The resulting alloy gives the silver strength. The standard is at least 92.5% silver.
Vermeil (verr-MEEL): A sterling silver that has been gold-plated.  A good combination for those with allergy to normal, plated findings. The difference between vermeil, and gold-filled, is in the thickness of the gold and the base metal used. In vermeil, the base is sterling silver. The standard is for the gold plating to be much thicker than ordinary plating.
Gold- filled: These are made of a base metal (usually brass or copper) covered by sheets of gold in a mechanical bonding process. Effectively a thick coat of gold: the gold content is 5% or 1/20 of the total weight.  Usually made with 14k gold, it is hard wearing. With reasonable care it will not peel or flake, and should last as long as solid 14k gold jewellery. It is safe for most people with sensitive skin.
Gold-plated: A base metal such as steel or brass is dipped into a bath of electroplating solution, with a lump of solid gold. When an electric current is applied, a thin layer of gold is deposited on the metal. Since the plating is quite thin, the plate (and hence the colour) on findings can wear off.

Pewter: A malleable metal alloy, traditionally 85-99% tin, with the remainder consisting of copper, antimony, bismuth and lead.

Rhodium: Rhodium is hypoallergenic. Rhodium protective coating is useful for white gold which often contains nickel to give it a white sheen look. It is well known that nickel is not biocompatible and approximately 15% of people suffer from some kind of allergies to nickel. Thus rhodium serves as an additional layer between skin and base metal preventing skin from irritation.

In additional to its physical properties, rhodium has a beautiful bright white look peculiar only to rhodium.