Moonstone is a composite mineral, made up of two different types of feldspar - orthoclase and albite. Moonstone is famed for its unusual shimmering, opalescent optical quality, called adularescence.  This phenomenon is a type of schiller somewhat similar to the labradorescence displayed by labradorite, another composite feldspar.
Depsite the name, moonstone does not have lunar origins; its name derives from its appearance. When moonstone was forming, the two types of feldspar were mixed. As the rock cooled, the feldspars separated into stacks of very thin, alternating layers. Variations in the light performance characteristics of the layers generate the optical effect and the separation of the light into different colors. 
Moonstone is most often fashioned as a cabochon (also nicknamed a "cab" or "cabbie") - with a domed top and flat underneath. When a moonstone is cut this way, the skilled cutter takes care to align the stone so that the optical shimmering effect is best presented. You may sometimes see an unusually transparent moonstone cut as a faceted gem - in which form it can have a uniquely magical beauty. 
Moonstone is mined in a number of places - including Sri Lanka, Burma, India, Australia, Austria, Mexico, Madagascar, Norway, Poland, Canada and the United States. 
Moonstone is somewhat soft, and so older stones can become dull through surface wear. It is possible to have them polished by a jeweler in order to bring back their luster. 
Moonstone has been categorized into two types: Adularia Moonstone, which is more transparent, and typically originates in Asian countries such as Burma and India; and Albite Moonstone, which is typically less transparent, and mainly originates from Canada.  You may occasionally find moonstone referred to as simply "adularia" or "selenite", but more typically, it is all "lumped" together and simply labeled as moonstone - and it is up to the buyer to discern the indentity and quality of what they are getting.
Moonstone prices are enormously variable. The highest grade of moonstone is colorless, perfectly clear and has a royal blue shimmer. Such stones are most often cut en cabochon and may be seen priced at around $100 / carat retail (March 2011). Once seen, these stones are not easily forgotten as they are in a class of their own. The simplest way to see a number of these stones for comparison is to do an eBay search for moonstone, select the "Loose Diamonds and Gemstones" category, and then arrange the list "Price + Shipping: Highest First." The bluish moonstones from Sri Lanka are considered to be some of the best quality - and are becoming expensive as they are becoming scarcer.  Lesser stones may have other colors, appear more milky or cloudy, and have inclusions.
Moonstone can occur in a few varieties other than the typical blue-white-silver-cream range of colors commonly seen. Some moonstones diplay more of a prominent "rainbow effect" and are called rainbow moonstones.  Occasionally one sees red, pink, orange or red-brown moonstones; however the black stone marketed as black moonstone is in fact a type of labradorite.
Moonstones with the chatoyant "cat's eye" effect are quite common - with a flash of light reflecting across the center of the cabochon, as seen in the second image above. However, you may also occasionally see a rare "star moonstone" - which displays asterism in a similar manner to a star sapphire or star ruby. The phenomenon of asterism, as the name suggests, displays a four or six-pointed star reflection within the stone when light shines on it. I have seen photos of moonstones for sale with both four and six rayed stars.
In folklore it is said to be a stone that inspires emotion, feeling and any "feminine qualities" - assisting intuition and giving benefit especially to young women and those who care for others. Folklore also states that it brings good fortune to lovers and those travelling by night or over water. In history, moonstone was believed to have bizarre "supernatural" qualities that do not make much sense, although they make interesting reading. See: https://gemstoneslist.com/blog/the-bizarre-history-of-moonstone-that-makes-no-sense/
Moonstone is often seen set into silver jewelry: In addition to the way their colors complement each other, the two materials, silver and moonstone, have a symbolic connection; with silver since ancient times being the metal associated with the moon.
Photo by Didier Descouens - released under CCA 3.0 Unported License
Moonstone (from Mexico, faceted - total weight 7.69 carats)
Image © supplied by Woodmansee* Gems
Moonstone - Sources Referenced:
 Edwin William Streeter - "Precious Stones and Gems: Their History, Sources and Characteristics", 1892
 "The Curious Lore of Precious Stones" - George Frederick Kunz (1913) p.97-98
 Oxford English Dictionary, 1971; "Selenite"
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