Found all over the world, moonstone is prized for its blue to white adularescence — a billowy, moonlight-like sheen. Despite being somewhat fragile, this alternative June birthstone is a popular choice for jewelry.
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Historically, the most valuable colorless, transparent moonstones with strong blue sheen came from Myanmar. Unfortunately, this material is essentially mined out. Most of the top-grade, blue sheen gems available today are being passed from one dealer or collector to another, which escalates the prices.
For cat’s eyes and the occasional star, expect to pay in proportion to the beauty, size, and clarity of the eye or star.
Like their evocative namesake, the Moon, moonstones have inspired quite a bit of romantic lore. People have associated moonstones with fertility, love, protection, and, of course, the Moon.
Jewelry lovers, especially royalty, have highly valued these gems for centuries. Currently, they’re probably most popular in Germany and the Scandinavian nations, where they’re preferred over pearls and alexandrites as June birthstones.
Adularescence is caused by the diffraction of light as it hits thin, alternating layers of orthoclase and albite within the gem. This produces the appearance of a floating, cloud-like, blue to white light inside the gem. A blue sheen is produced if the albite crystals are very fine. (Fine orthoclase and albite plates are dispersed within each other as a result of unmixing on cooling). If the albite plates are thick, the sheen is white.
Generally, lapidaries cab moonstones with high domes to accentuate adularescence. Specimens with strong displays often reveal cat’s eyes when cabbed in this manner. Rare asterism, when it occurs, produces four-legged stars.
The body color of an orthoclase moonstone is generally due to its iron content and may be white, beige, brown, red-brown, orange, greenish, or yellowish. Goethite (iron oxide) inclusions will cause red coloration.
A moonstone variety containing the orthoclase feldspar sanidine occurs in Grant County, New Mexico, United States.
What is a “Rainbow Moonstone”?
Some transparent plagioclase feldspars, such as labradorite, also have thin layers of albite. These also produce a blue schiller effect if thin and a white effect if thick.
In rare instances, a multicolored schiller displays blue with green and/or orange colors, a phenomenon known as labradorescence. Although such stones are often called “rainbow moonstones,” they’re technically a variety of labradorite, not moonstone. Labradorescence is distinct from adularescence. However, the gem trade has generally accepted the use of the name “rainbow moonstone.”
Moonstones are one of the few gems that have inclusions so characteristic that seeing them guarantees their identity. They contain fissure systems along incipient cleavages in the body of the material created by exsolution pressures. Such fissure systems are short pairs of cracks, running parallel to the vertical axis of the crystal, with shorter cracks emanating perpendicularly along the length of the parallel fissures. These resemble many-legged insects under the microscope and are known as “centipedes.”
Moonstones also have rectangular dark areas due to stress cracking or cavities. Sometimes, a cavity extends from such a rectangular dark area, creating a comma-shaped inclusion.
Myanmar moonstones characteristically have oriented needle inclusions.
Specific Gravity and Refractive Index
Material from Sri Lanka tends to have specific gravity values on the low end of the moonstone scale, 2.56. Material from India tends toward the high end, 2.59.
Most moonstones usually have refractive indices (RI) of 1.520-1.525 with a birefringence of 0.005. However, material from Virginia has an RI of 1.518-1.524 with a birefringence of 0.006.
Although moonstone has been simulated by milky chalcedony and certain types of synthetic spinel, these substitutes usually look inferior and are easily spotted. Lab-created moonstones haven’t entered the market.
Historically, Myanmar has produced the finest material.
Other notable sources include the following locations:
Australia; Austria; Finland; India; Madagascar; Mexico; Norway; Sri Lanka; Switzerland; Tanzania.
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 usually well-cut and very attractive. Moonstone with a blue sheen, the most valuable kind, rarely occurs in sizes over 15-20 carats. However, stones with a silvery or white adularescence are abundant and available in sizes up to hundreds of carats.
Discovered by the first Japanese expedition to Mt. Kilimanjaro in 1918 and reputed to weigh between 300 and 450 carats, this might be the largest known moonstone. Photo by Mr Matthew Hardy Japan. Licensed under CC By-SA 3.0.
How to Clean Moonstones
Although popular jewelry stones, moonstones have a hardness of 6 and a slight tendency to chip and cleave. They should receive protective settings, especially for ring use, to prevent scratching. Brooches and pendants will minimize exposure to hazards, but, in any jewelry setting, protect moonstones from hard knocks. Don’t use mechanical systems like ultrasonic or steam for cleaning. Instead, use only warm, soapy water and a soft brush.