The moonstone is the only one that has a glow whose interesting azure whiteness and excessive brilliance can only be compared to the glowing effect of the full moon." »Wrote, around 1825, Antoine Caire-Morand, famous lapidary, in his work devoted to precious stones.

Formerly designated by different names: lunar, fish eye, specular, argentine, it is now classified in orthoclases under the name adular. A very austere label for this beauty of mother-of-pearl and silver, so we prefer the delicate name of "moonstone".

Mineralogical characteristics of moonstone
Moonstone is a mineral of the silicate class and the tectosilicate subclass and feldspar group. There are two families of feldspar:

• alkaline feldspar (potassium): orthoclase, microcline, sanidrine, etc.

• calcium-sodium feldspar called plagioclases: oligoclase, albite, labradorite ...

The name "moonstone" is usually given to a certain variety of orthoclase (or orthosis) with white, blue or silver iridescence: the adularia. However, other feldspars having the same appearance can also be moonstones, e.g., white labradorite, sanidrin, oligoclase, albite ...

The adularia comes originally from the Swiss Alps. All the orthoses with the characteristics of this variety can today carry this name, wherever they come from. It is found in granites, pegmatites, hydrothermal veins, in acidic and alkaline magmatic rocks.

Beautiful and large specimens are very rare. The most popular adulars are colorless or white with blue or gray-blue reflections. Those with yellowish reflections are less popular. The adularia presents a perfect cleavage, it is transparent to translucent with a vitreous or silky luster. Quite hard, 6 / 6.5 on a scale of 10, it is brittle and fragile.

Adularescence

This is the name given to the phenomenon of iridescence specific to the adularia but also to other similar stones. Adularescence arises from the reflection of light between the myriads of fine lamellae developed during the crystallization phase.This particular shimmer gives silver reflections that appear to float on the surface of the stone. This characteristic is also called the “Schiller effect”. The adularescence is revealed by a cabochon cut, the pearly waves then move gracefully under a light source. The adularescent effect disappears at a temperature of over 750 °.

Blades of albite can give distinctly milky undertones. Some extremely rare stones have a star effect (asterism) due to tiny inclusions of rutile or ilmenite. Exceptional moonstones can be admired at the Smithsonian Institute in Washington.

Extraction sites and lodgings

The most beautiful moonstones are quarried in the southwest of Sri Lanka. It is also found in the following countries and places:

• India (Tamil Nadu), where sanidine is common, generally more colorful and satin in appearance
• Burma
• Madagascar (Itrongahy), for yellow orthosis or sanidine
• Tanzania
• Canada (Mont Saint-Hilaire)
• Brazil
• Mexico, where we find sanidine or adularia called valencianite
• United States (State of New Mexico)
• Russia (White Sea)
• Australia
• Austria (Tyrol), Switzerland (Saint-Gothard)
• France (Cantal, Haute-Vienne, Hérault, Lozère, Puy de Dôme)

Confusion and deception

Confusions are possible with:

• girasol quartz
• white chalcedony
• opal
• selenite.

Rainbow moonstone, Norwegian moonstone, and spectrolite are Labradorites. The name "Californian moonstone" is prohibited since it actually designates a chalcedony.

There are fake moonstones. The doublet technique and the colored coating of stones then called “celestial moonstones” are fallacious practices aimed at artificially intensifying colors and reflections. We try to imitate moonstone by heating colorless synthetic spinels to obtain a milky appearance.
Are there "moon stones" on the moon?

A large number of minerals from the same family are present there: plagioclase (sodi-calcium) feldspars in particular, potassium feldspars exist more rarely. The astronauts brought back more than 300 kg of lunar minerals. At the Cape Canaveral Space Center in Florida, visitors can touch a real moonstone. In France, one can be seen at the Cité de l'Espace in Toulouse weighing 2 kg. The appearance of these specimens is not very attractive and in no way evokes the resplendent star of their origin, unlike terrestrial moonstones.

Etymology and meaning

In 1801, René-Just Hauÿ proposed the name orthosis to designate the mineral of which moonstone is a variety. From the Greek orthos meaning "right", this term evokes the cleavage at right angles of this mineral. The international term chosen is orthoclase.

At the same time, the French mineralogist, Jean-Claude Delamétherie gave the name of hecatolite to the pearly feeldspath inspired by the Greek goddess of the moon Hecate.
Hecatolite quickly gave way to the name adulaire in reference to the place of its discovery. Adulaire comes from Mount Adule or Adula in the Saint-Gothard massif (Swiss Alps). The Greek geographer Strabo, born in 64 BC, was the first to name this place in his books. A century later, another Greek scholar, Ptolemy, thinks and writes that the Rhine has its source “in Adula Monte”.

Moonstone through history

Moonstones in Antiquity

We have several descriptions that may match our current moonstones. These stones certainly came from the East or even from the Alps:

• Argyrodamas: silver colored stone.
• The asteria: it keeps locked up a certain mobile light like pupils.
• The astrion: white, in the center shines like a star whose light resembles that of the moon.
• The astrobal: resembling the eyes of fish. Exposed to the sun, it throws white rays.
• Astroïtes: also white, with the magical virtues employed by Zoroaster.
• The céraunie: crystalline with blue reflections, it absorbs the light of the stars. Sought after by mages, it can only be found in places where lightning strikes.

Finally, there is selenite or selinitis (name of Dioscorides evoking the moon goddess Selene): whitish, transparent, whose luster can have the color of honey. It renders, like a mirror, the figure of the moon depending on whether it is in its crescent or in its decline. Today, selenite is a mineral classified in gypsum, but in Antiquity and until the Middle Ages, this name seems above all to designate a moonstone with magical powers.

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