Moonstone is the best known gem variety of orthoclase feldspar, being potassium aluminium silicate. It is usually polished as a cabochon, and is often carved with a moon face. Its importance as a gemstone arises because of its schiller, which in moonstone has been given the name adularescence.
This is caused by a feature of its crystal structure. Orthoclase feldspar and albite are present in close association, arranged in layers. This causes an interference effect of light. The colour of this schiller regarded as most desirable is blue, which usually occurs in a white or grey body colour. If the albite layers are too thick this schiller appears whitish which is less attractive.
Probably the most famous moonstone of all was not a moonstone but a diamond. In the well-known classic book "The Moonstone" by Wilkie Collins, a large diamond was stolen from an Indian shrine, and a retired policeman is hired as a private detective to investigate a murder and robbery. Wilkie Collins is recognised as the inventor of the English detective novel, and was a friend and colleague of Charles Dickens. Although "The Moonstone" was written in 1868, it remains to this day a masterpiece of English literature.
The book is clearly based on a number of stories about famous diamonds which have disappeared and reappeared over the centuries.
Inclusions
Moonstone contains a number of typical inclusions which appear to be stress cracks. These give the appearance of centipede type insects, Chinese "aeroplanes" looking like characters from Chinese writing, negative cavities looking like rectangular crystals, and needle-like shapes.
If present in large numbers, the needle shaped inclusions can cause a cat's eye chatoyancy effect.
Colour:
The most desirable colour of moonstone is blue, but it also occurs in grey, white, pink, green and brown.
