BLUE JOHN

It is a variety of flourite and is also known as calcium flouride (CaF2). It is a member of the halide group and is relatively soft (4 on the mohs scale). It is a colourless mineral but may have bands of blue, purple, white and yellow due to inclusions.

It has been extracted since ancient times and today is used mainly for jewellery and vases. It is no longer used in industry because the main sources are now exhausted.

Now it is only known to exist in Blue John Cavern near Castleton in the Peak District (Derbyshire, England). The caves here are limestone. The calcium flouride of Blue John formed when the original rocks were altered by hydrothermals which contain flourine particles.

BOWENITE

Bowenite is a variety of antigonite (a member of the serpentine group). It is a silicate of magnesium and ferrous iron and is usually green. It is very soft (2.5-3.5 on the mohs scale) and therefore is easily scratched. It closely resembles nephrite and may also be used to simulate jade.

It is named after G. T. Bowen, the U.S. mineralogist who first found it in 1822.

Bowenite is found in igneous rocks and has a greasy feel. It is a secondary rock and is formed by the alteration of serpentinite minerals e.g. olivene.

One of the most important sources is at Rhode Island (US). It can also be found in Italy, California and Utah.