In this article we talk about a beautiful gem – the moonstone or the adular.”Moonstone” is a commercial name that combines a whole family of feldspar minerals with adularescence effect. This optical effect also known as irisation. It comes when the crystal is correctly oriented during the cabochon cutting process and looks like a pearl white or colored flare of light on the surface. The iridescent feldspars are diverse in composition and differ in structural features and appearance.
They are divided by chemical composition into potassium (microcline, orthoclase, sanidin) and sodium-calcium feldspars (andesine, labrador, plagioclase, oligoclase, sunstone, albite).
Classic gem moonstones of high quality (from almost transparent, translucent, with a minimum content of inclusions with blue irisation) in most cases are the mineral orthoclase, specimens with a rainbow play of light is plagioclase. Next, we will consider orthoclase and plagioclase as the most prominent representatives. They are also called adular, in honor of Adul’s place in the Alps in Switzerland, where one of the first discovered deposits is located.
A characteristic form for moonstones is cabochon. Almost 99 percent of the extracted crystals are processed in round or oval cabochons. There are reasons for this. The first is the difficulty of polishing flat surfaces, complicated by cleavage characteristic of a mineral. The vast majority of instances are translucent, and look winning in faceted form. And finally, the iris is most beautiful and spectacular in the cabochon.
The hardness of the adular is low, but it is considered sufficient for daily wearing in the jewelry, subject to a careful attitude, in addition to irisation in adular, also known as adularescence. In some samples the optical effect of the cat’s eye can be observed and much more rarely asterism. It should be noted that all optical effects appear only with a bright point source of light.
Feldspar minerals of various compositions are mined in many mines of the world. Here we will talk about the most valuable varieties. It is generally accepted that the best adularescence orthoclases are usually associated with Sri Lanka deposits, but today the situation has changed, and good moonstones from Ceylon practically do not enter the market. Another historical mining site is India. In silver jewelry there are Indian moonstones with bright irisation, but, unfortunately, with many internal heterogeneities. The main source of adular today is Africa, Tanzania in particular. Rainbow specimens recently appeared in small numbers from some dealers and immediately bought by collectors of rarities are mined in Madagascar. Burma also has moonstone mines.