The pearl is one of the oldest precious gems known to man, but became less exotic when cultivated pearls arrived on the market. Other organic gems include amber, coral, ivory and jet.
Used in jewellery from the 19th century onwards, platinum is prized for its white brilliance and malleability. Gold, silver and platinum are collectively termed precious metals and traded on the world markets. Britain recently caused a dip in gold prices when Chancellor Gordon Brown decided to sell national gold reserves. At current prices, though, expect to pay around pound;163 for an ounce of gold, pound;3 for an ounce of silver and pound;223 for an ounce of platinum.
After gold, silver is the most widely used precious metal for jewellery. It first appeared in jewellery in Greek times. Traditionally, it was used only for economic reasons, or to bring out the light in transparent stones. It is now most jewellery artists' preferred metal.
Only diamonds, rubies, emeralds and sapphires are classified as precious stones, although the topaz, chrysoberyl and zircon are considered 'important' stones. Semi-precious stones include amethyst, garnet, aquamarine, turquoise, jade, opal, lapis lazuli and malachite.
The diamond is the hardest gemstone. The highest price ever paid for one was $10.5 million for a 100-carat, pear-shaped stone, purchased from Sothebys by a Saudi sheikh in 1995.
All the gold ever mined would easily fit under the Eiffel Tower, forming a cube 19 metres each side. Valued for its strength and malleability, gold used to be a rare metal. But the gold rushes in California, the Yukon and Australia in the mid 19th century, combined with the development of machines for making chains, have made to gold becoming a mass-market product. These days, all newly-mined gold is turned into jewellery.
The gold in most jewellery is mixed with other metals, such as silver or nickel. But in some parts of the world, especially the Middle East, India and south-east Asia, pure gold is purchased as an investment.
Diamonds and all other precious stones are weighed in carats. The word comes from the carob seed, which was used to balance scales in oriental bazaars. Until t he universal adoption of the metric carat (200mg) this century, 'old' carats varied in weight - some weighed up to 206mg. Gold is also measured in carats, but the system is different as caratage indicates the proportion of pure gold in jewellery - 24-carat is 100 per cent gold and all other gold is measured in parts of 24. Thus 22-carat is 22 parts gold and two parts alloy.