Stone Mania

A gemstone is a precious or semi precious mineral, rock or stone that can be cut (faceted) and polished for use as a gem.  As well as being used in jewellery, gemstones are also used for wide variety of other purposes as well.  Among the most highly valued and well known gemstones are Diamonds, emeralds, rubies and sapphires.  Many of the most prized gemstones are found in museums or private collections.  

Most gemstones are minerals, which means they are naturally occurring materials with a particular chemical composition and crystalline structure.  In terms of chemical composition, the largest group of gemstones consists of silicates and the second largest is oxides.  Emerald is a silicate of beryllium and aluminum, otherwise known as beryl and rubies and sapphires are composed of aluminum oxide otherwise known as corundum.  Diamonds, are a crystalline form of carbon.

Some gemstones such as lapis lazuli are classified as rock meaning they are composed of a mixture of minerals.  Others are made of organic matter, for example pearl is produced by oysters, amber is fossilized tree resin and jet is a form of coal.  If a gemstone is mainly made up of one single mineral but contains small but visible quantities of a different mineral or fossil, those 'flaws' are known as inclusions.

The value of a gemstone is usually determined by its beauty, rarity, durability, size and often shape is also taken into consideration.  Other characteristics which are also important are colour and any unusual optical effects such as chatoyancy, iridescence or play of colour.  Gemstones are also cut (faceted) to enhance their colour and/or optical features.   

The following link offers further reading on gemstones

http://en.wikipedia.org/wiki/Gemstone