ICELAND SPAR

A member of the carbonate group which is the pure form of calcite. It is very soft (3 on the mohs scale) and has an amazing optical effect. When light passes through Iceland Spar it splits into two rays which shine in different directions. This is known as double refraction or birefringence. Although evident in all forms of calcite it can be seen more clearly in Iceland Spar because it is transparant and colourless. It is for this reason that Dutch physicist Christiaan Huygens used it to formulate the laws of double refraction and develop the wave theory of light. Crystals form in rhombus shapes and it may give off red, pink or blue light when placed under an ultra violet lamp (flourescence). It is also known as Iceland Crystal.

It forms in sedimentary and metamorphic rocks with some crystals growing to very large sizes. The largest specimen was found in Iceland measuring 7m x 7m x 2m, hence it's name. However it can also be found in Germany and Colorado (US).

 

IOLITE

A silicate which is usually violet but may also be yellow or grey. It is relatively hard (7 on the mohs scale) and derives it's name from the Greek word ion which means violet. However it is also known as Cordierite after the French geologist Pierre Cordier who first described it in great detail. Less commonly it is sometimes called dichroite which orinates from the Greek word meaning two coloured. This refers to it's ability to change colour depending on the angle from which it is viewed which is a characteristic known as pleochroism. This enhances its beauty and is also useful for practical purposes. The Vikings used it as a navigational aid in order to find the the position of the sun on overcast days. This is why it is sometimes known as Vikings compass.

Iolite is very widely distributed and therefore not very valuable. Due it's optical properties it is rarely produced synthetically. The finest specimens may be faceted to bring out its natural pleochroism whereas lower grade iolite is often polished or used for beads. Very low grade iolite specimens which are very pale purple in colour are known as water sapphires.

It is usually found in pegmatite rocks or alluvial deposits in the form of small pebbles. Deposits can be found in India, Madagascar, Sri Lanka, Finland, Germany and Conneticut (US).