CHRSOPRASE

Chrysoprase is a green variety of chalcedony quartz (also known as prase). Like quartz it is composed of one atom of silicon and two of oxygen and is therefore a form of silicon dioxide.

It is green due to the presence of small particles of hydrated nickel sulphate. This becomes incorporated into the gemstone when it first solidifies and is usually spread evenly throughout it. Although it is fairly hard (7 on the mohs scale) it is brittle and it's colour may fade after a long time of exposure to the sun.

Chrysoprase is mainly used for jewellery. It's green colour is sometimes enhanced in order to increase it's value. It may be substituted by yellow chalcedony which can be dyed green with chromium or nickel salts.

The finest samples are found in Australia but it may be also be found in Poland, Czech Republic, Russia, California, Brazil and Austria.

CITRINE

A very common form of quartz which is usually yellow but may also be brown/orange. It is fairly hard (7 on the mohs scale). It derives it's yellow colour from oxides of ferric iron but apart from this it is almost free from inclusions. It's colour is usually consistent throughout.

It forms in many parts of the world when quartz melts and then resolidifies with inclusions of iron. It's name is derived from the Greek word kitros which means lemon. It is usually classed as an oxide but is also a silicate because it is an oxide of silicon.

Although it rarely has a strong enough colour to be used for jewellery if it is heated it may become pink and can then be used to simulate topaz. Citrine may itself be simulated by amethyst because when this is heated it changes colour from purple to yellow. (It is this type of citrine which is usually found in shops in the UK) This would not have occurred years ago because amethyst was more valuable. It's value declined dramatically after the discovery of amethyst in Brazil and Uruguay at the end of the 19th century.

Some gemstones are made up of a naturally occurring combination of amethyst and citrine. These are known as ametrines.

It may be found in igneous, sedimentary or metamorphic rocks. It may be found in Brazil, France, US and Russia.