Gem Focus – June 2019

Posted on June 1, 2019 by The Gemworld Staff


Legendary gemologist G.F. Kunz in his “The Curious Lore of Precious Stones” writes that moonstone is believed to bring good fortune and regarded as sacred in India so much so that it is never displayed for sale, except on a yellow cloth for which yellow is a sacred color too.

The billowy sheen in moonstone is actually a phenomenon known as “adularescence” which is created by diffused reflection and scattering of light as it moves from alternating layers of feldspar. Feldspars are a group of complicated aluminosilicates which crystalize in a solid solution series. They are the most common mineral in the earth’s crust. Feldspar species have been known and used since ancient times for personal adornment as well as for industrial purposes, especially in glass manufacturing. There are 26 species of feldspars, nine of which are known to be found in gem quality. Transparent feldspar species, i.e., transparent orthoclase and labradorite are more commonly sought as collectors’ stones and seldom for jewelry since they are not very durable.

They tend to be colorless, light yellow, sometimes gray. The unusual variety of microcline feldspar, which displays an attractive greenish blue color is called amazonite. The common transparent feldspar species become more desirable when they display phenomenon such as adularescence (moonstone effect), labradorescence (blue, green, yellow flashes of light caused by interference through twinning planes), aventurescence (sunstone effect), chatoyancy and asterism.

Traditionally, moonstone is a variety name for orthoclase-albite feldspar with adularescence. The most desirable ones display transparent body color with blue sheen. Moonstone body color can be white, light green, yellow to brown, or gray and the adularescence colors can be gray or white. There is also “rainbow moonstone” in the market that is not orthoclase-albite. Rainbow moonstone is colorless and transparent labradorite feldspar displaying strong labradorescence. Purist gemologists would argue this terminology but the name is widely accepted and used in the gem market since rainbow moonstone is highly desirable and more expensive. Fine and transparent examples of moonstone may be faceted but due to feldspar’s low hardness, they don’t display the best brilliance. Most moonstones are cut as high cabochons to display the adularescence in more dramatic ways. Some stones might even display additional phenomenon such as chatoyancy or asterism.

Prices for feldspar gems such as moonstone and sunstone are fairly stable allowing for the consistency that manufacturers look for when developing a new line. However, exceptional pieces are priced individually in a few hundred dollars per carat or more. There has been an increased popularity of rainbow moonstones in the market and the per carat prices has risen in a way that feldspars never experienced before. While commercial grade moonstones are sold at a few dollars per carat, fine and extra fine quality moonstones in larger sizes are traded at hundreds of dollars per carat priced at the wholesale level.

Download the Full Article