In moonstone, adularescence is due to a layer effect, where thin inner strata of two types of feldspar intermix, (exsolution regions of sodium feldspar in potassium feldspar). These layers scatter light either equally in all spectral regions producing a white shiller, or as in the most valuable specimens, preferentially in the blue or the blue and orange. As in so many cases of optical phenomena the size or distance from layer to layer influences the colors we see. Very thin layers produce blue "shiller" and thicker layers produce white. Body color is generally due to iron content. That the adularescent glow of light appears to come from below the surface , distinguishes it from other superficially similar optical phenomena like orient and simple iridescence. The term "shiller" or "schiller" is sometimes used to describe the light.
Stones with strong shiller due to a specific, internal organization oriented along specific crystal axes, and which are skillfully cut to display it may show the additional phenomena of the cat'seye effect or more rarely asterism. Unlike corundum and several other gem species with six-rayed stars moonstone is characterized by a four ray star with one arm of the "cross" stronger than the other.
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.
Identifying moonstone is generally pretty easy as nothing else really looks like it. If necessary the refractive index and/or a microscopic examination can be counted upon to distinguish the genuine article. Moonstone is one of the relative handful of gems that have microscopic 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.
Moonstone has been simulated by milky chalcedony, and certain types of man-made spinels, but these substitutes are visibly inferior, and easily spotted. Synthetic moonstone has not entered the market, and enhancements are rarely encounted, there is one issue to be aware of though:
Large quantities of a highly fractured, translucent white Labradorite which originates in India is widely sold in catalogues and retail stores under the misnomer "rainbow moonstone" at very modest prices. (True moonstone is a MUCH rarer and considerably more expensive, species of feldspar. As you can see, the material in question is no less attractive for bearing its improper name.
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 (about the same care as one would exercise with opal or pearl). Brooches, earrings and pendants are quite safe for daily wear, however . Other than refraining from cleaning with steam or ultrasonics and protecting the stones from hard knocks, no special care is necessary.
In general, a moonstone is more valuable the more transparent it is, and the more blue its adularescence. 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'seyes and the occasional star expect to pay in proportion to the beauty of eye, size and clarity. In rare instances, the shiller 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, though, 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.
For some gems which often come in large sizes of good quality (like colorless or yellow beryl and smokey or rock crystal quartz) little premium needs be paid for a larger gem. This not the case with many gems (sapphires, emeralds, rubies, Benitoites, etc. etc.) which are so rare in large size and good quality that the price per carat escalates not linearly, but exponentially. You can expect to pay very high prices for large moonstones as they are one of Earth's least commonly produced treasures.
