Moonstone is a mineral with a hardness of 6 out of 10 on the Mohs scale of mineral hardness [?]. These Monoclinicly structured gems are made of potassium aluminum silicate, their full chemical compound being KAlSi3O8.
The variety name Moonstone is usually used to describe an optical effect and unlike most variety names it is not confined to a single species (The term is also applied to albite-moonstone, microcline-moonstone, labradorite-moonstone).
But Moonstone most prominently refers to the orthoclase feldspar, Adularia Moonstone, a microperthitic association of orthoclase and albite) and rarely to Albite Moonstone, the sodium-rich end-member of the plagioclase feldspars.
Although feldspar is the most abundant mineral group on Earth accounting for around 50-60% of the Earth's crust, gem-quality feldspars are rather rare, and Adularia Moonstone is a gem-quality variety.
Moonstone generally has an almost transparent ground, which is practically colorless, pale grey, or tinged with yellow, with a whitish to silvery white or blue shimmer. The body colour can range from colourless to grey, brown, yellow, pale orange or peach, green, plum-blue, reddish or pink. The best moonstone has a blue sheen, perfect clarity and a colourless body colour. This material has become increasingly rare and the term "blue-sheen" has been stretched to include gems of lesser quality.
Because of its slightly turbid transparency, gems cut en cabochon show a mobile reflection, and even a cat's eye effect or chatoyancy, which is softer and more diffuse than that of other chatoyant stones. When cut cabochon, it has a sheen known as adularescence, which is caused by internal alternate layers of albite and orthoclase of stone, which spread the light falling on the surface of cabochon. The reflected rays from the surface of stone are bluish to whitish silvery schiller, which is called Rayleigh scattering, more common on stones having thicker structure layers.
Moonstone's adularescence is its most distinctive quality. Top quality fine blue moonstones show an incredible "three dimensional" depth of colour. Its counterpart in the plagioclase group, albite moonstone, can look identical for the same reason, but the two may be distinguished by their density, which is lower in orthoclase.
The moonstone effect is generally attributed to the interference of light reflecting from submicroscopic lamellar structures (comprising two types of feldspar) producing iridescence. But experts agree that more than one phenomenon and several factors contribute to the moonstone effect.
The most prominent source of moonstone is in the gravels and pegmatites of the Dumbara district, Sri Lanka. Many gem moonstones come from India, Brazil and Madagascar. Other localities include Switzerland, Tanzania, Pennsylvania, New Mexico Colorado, Indiana, New York, North Carolina, Pennsylvania, Wisconsin and Virginia in US, and Myanmar, Canada, Australia and Russia.
Much adularia moonstone has come from New Mexico. Brazil pegmatites, some K-feldspar (potassium-feldspar) crystals, roughly 3m long have been reported. Brownish transparent crystals around 5 cm have been reported from Greenland.
Substitute materials that can nearly imitate this effect are opalescent glass, white chalcedony that exhibits a light blue schiller, milky quartz (when cut cabochon), synthetic spinel that has been reheated, or some amethyst showing moonstone effect through heat-treatment. Black moonstone is a misnomer for darkened variety of labradorite from Myanmar, pink moonstone is a misnomer for scapolite, spectrolite from Finland is actually labradorite. Also misnomered as water opal.
Oregon moonstone is a misleading term for chalcedony.
Moonstone is also called girasol, belomorite, hecatolite, water stone, Ceylon opal.
Moonstone glass is an opal glass that resembles moonstone.
Moonstone gets its name from its appearance which is likened to the reflected shine produced by moonlight. It is normally characterized by a milky white to bluish shimmering sheen effect that seems to float across the convex surface of the stone as it is turned and moved: the more intense the colour and the larger and more transparent the stone, the more valuable the gem.
Moonstones are almost always cut cabochon. It is also cut into curved pieces for threading into necklaces. Incipient cleavage cracks may be visible inside the stone.
Its gem value is fairly low, but the type with a blue reflection is quite highly prized.
An imitation form consisting of synthetic spinel is muck milkier in appearance, without a mobile reflection. Adularia moonstone has not been manufactured synthetically.