COLEMANITE

A member of the borate group. It's crystals are usually clear or colourless but it may appear grey, milky white or yellow. It has perfect cleavage and a glassy sheen but gem quality crystals are rare.

It is fairly soft (4.5 on the mohs scale) but much harder than most other borates. It is an important source of boron which was first discovered in England by Sir Humphrey Davy. Boron is very heat resistant and is used to make glass for chemical apparatus such as test tubes and petri dishes. Boron produces a green flame which makes it useful as a component for flares. It it also used in deodorants..

Colemanite was first discovered in Death Valley California in 1882 and was named after William T Coleman who was the owner of the mine where it was first found. Large deposits were found and mining towns grew up nearby, one of which was named boron.

Colemanite is never found on it's own in nature and is always part of a compound. It is a secondary mineral and is found in deposits formed by outpourings of volcanic gas or in dried up lakes of borate salts in deserts (playas).

As well as Death Valley California it can also be found in Nevada USA, Argentina and Turkey.

CORUNDUM

An aluminium oxide which is insoluble. It is colourless when pure but may contain some iron or titanium which can colour it red, blue, yellow or green. Therefore, it may appear to be a variety of colours due to the inclusion of impurities. If it contains no impurities and is colourless it is known as colourless sapphire (this is very rare).

The most well known forms of corundum are ruby (red corundum) and sapphire (blue corundum). Ruby gets it's red colour from the presence of chromium and titanium. Sapphire is blue due to the inclusions of iron and titanium.

It is the hardest substance on earth after diamond (it can only be scratched by diamond). Due to this it is often used as an abrasive (for grinding, polishing and on emery boards). It may also be used for clock bearings or mined as a source of aluminium but is most valuable for it's use as ruby and sapphire. Crystals are usually twinned. (A lens is needed to see this)

Corundum was first discovered in Europe in the late 18th century. It forms in metamorphic rock and igneous rock where there is little silica present.

It may be found in the Ural Mountains, Canada, USA, Australia, Burma and Sri Lanka.