Moonstone or "Adularia", an orthoclase feldspar, was originally named for an early mining site at Mt. Adular in Switzerland. From this tradition we derive the term "adularescence" which is the optical phenomenon of iridescence which creates a billowy, floating blue to white light in this gem. Adularescence is due to diffraction of light as it hits thin, alternating layers of orthoclase and albite within the gem. Very thin layers produce blue "schiller" and thicker layers produce white. Body color is generally due to iron content. Moonstone is one of the relative handful of gems that have inclusions so characteristic that seeing them guarantees the identity. Pairs of tiny stress cracks running parallel to the vertical axis of the crystal with smaller cracks aligned along them, have been called "centipedes" and are diagnostic of moonstone.
In general, the more transparent it is and the more blue its adularescence, the higher the value. Large quantities of near opaque material with various body colors is carved into simple "moon faces" and other figures which are available for pennies. Cabs of translucent material which are either white or with pleasing body color and adularescence are fairly common in the market and command relatively modest prices.
For cat's eyes and the occasional star, expect to pay in proportion to the beauty of eye, size and clarity. In rare instances, the schiller phenomenon is multicolored showing blue, with green and/or orange. Such stones are quite valuable and are known as rainbow moonstone. Unfortunately, a lot of low grade, labradorite feldspar is advertised and sold by that name, (a misnomer,) so most people are surprised by the prices of the real gem grade rainbow stones.
By far the most valuable moonstones are those which are colorless, transparent and have a strong blue sheen. Such stones historically came from Burma. Unfortunately this material is essentially mined out, so most top grade, blue sheen gems available today are being passed from one dealer, or collector, to another and prices are escalating.
The International Gem Society (IGS) has a list of businesses offering gemstone appraisal services.
|Is a Variety of||Orthoclase|
|Refractive Index||1.508 - 1.536|
|Hardness||6 - 6.5|
|Specific Gravity||2.55 - 2.61 - 3.35|
|Cleavage||Perfect and easy in two directions|
|Enhancements||Dark coating on back, enhances adularescence; uncommon; easily scratched; detect with magnification.|
|Special Care Instructions||See text|
Comments by Don Clark, CSM IMG
Moonstone or “Adularia”, an orthoclase feldspar, was originally named for an early mining site at Mt. Adular in Switzerland. From this tradition we derive the term “adularescence” which is the optical phenomenon of iridescence which creates a billowy, floating blue to white light in this gem. Adularescence is due to diffraction of light as it hits thin, alternating layers of orthoclase and albite within the gem. Very thin layers produce blue “schiller” and thicker layers produce white. Body color is generally due to iron content. Moonstone is one of the relative handful of gems that have inclusions so characteristic that seeing them guarantees the identity. Pairs of tiny stress cracks running parallel to the vertical axis of the crystal with smaller cracks aligned along them, have been called “centipedes” and are diagnostic of moonstone.
Gems range from colorless through shades of yellow, gray, green, and pink to brown and from transparent to opaque. Moonstone is a historically important gem having been valued, especially by royalty, for centuries. Its current popularity is highest in Germany and Scandinavia where it is preferred over pearl as the birthstone for June.
Generally moonstone is cabbed with a high dome which accentuates the adularescence. Those specimens with strong displays often reveal cat’s eyes when cut this way. Asterism is rare in moonstone, but when it occurs the star is four legged. Very rarely, extremely translucent to transparent stones are faceted. Although moonstone has been simulated by milky chalcedony and certain types of man-made spinels, these substitutes are visibly inferior and easily spotted. Synthetic moonstone has not entered the market.
This gem is so lovely that many people enjoy wearing it in jewelry. Caution must be used in setting and wearing rings or bracelets with this gem, due to the hardness, 6, and slight tendency to cleave. Brooches and pendants are quite safe. Other than refraining from cleaning with steam or ultrasonics and protecting the stones from hard knocks, no special care is necessary.
Comments by Dr. Joel Arem
Moonstone refers to feldspar of widely varying composition and from a wide variety of localities. The basic attribute is the presence of finely dispersed plates of one feldspar within another as a result of unmixing on cooling.
Orthoclase moonstone consists of albite within an orthoclase matrix. A blue color is produced if the albite crystals are very fine; the sheen is white if the albite plates are thick. The color of the orthoclase may be white, beige, brown, red-brown, greenish, or yellowish. Red coloration is due to goethite (iron oxide) inclusions. Some of this material cuts fine catseyes, where the sheen is concentrated into a narrow band. The sheen in moonstone is referred to as adularescence.
The density of such material is 2.56-2.59; material from Sri Lanka tends to be at the low end of this range, material from India at the higher end. The refractive index is usually 1.520-1.525; birefringence 0.005.
The moonstone from Burma and Sri Lanka is adularia and displays a white to blue sheen. The body colors may be white, blue, or reddish brown. The blue-sheen material, especially when the body of the moonstone is colorless and transparent, is very rare and greatly prized in large sizes (over 15 carats).
Grant County, New Mexico produces a very fine quality sanidine moonstone with a blue sheen. Orthoclase moonstone from Virginia is of a quality comparable to the Sri Lankan material: indices 1.518-1.524; birefringence 0.006. Moonstone also comes from Tanzania and several localities in the United States.
The name moonstone alludes to the lustrous sheen of this material, in the same way that sunstone derives its name.
Moonstones are characterized by fissure systems along incipient cleavages in the body of the material created by exsolution pressures. Such fissure systems are short parallel cracks with shorter cracks emanating perpendicularly along the length of the parallel fissures. These resemble many-legged insects under the microscope and are known ascentipedes. Moonstones also have rectangular dark areas due to stress cracking or negative crystals. Sometimes a cavity extends from such a rectangular dark area that creates an inclusion with a comma shape. Burmese moonstones are characterized by oriented needle inclusions.
Moonstone is rare in both large size and fine quality, but Indian material with strong body color is abundant and very inexpensive. This is fortunate because the material is well cut and very attractive. Moonstone with a blue sheen is the most valuable and is rare in stones over 15-20 carats. Stones with a silvery or white adularescence are abundant and available in sizes up to hundreds of carats.