Moonstone
Moonstones are prized for their adularescence, an optical phenomenon that creates the appearance of billowy clouds of blue to white light with a moonlight sheen. These feldspar gems have varying compositions and are found all over the world. Typically, they’re orthoclase feldspars composed of alternating layers of orthoclase and albite. Like their evocative namesake, the Moon, they’ve inspired quite a bit of romantic lore. Despite having a low hardness and a tendency to chip, moonstone is a popular choice for jewelry. In protective settings, its beauty can shine forth. Cabochons and carvings are the most common cuts for moonstone, though on rare occasions some transparent stones are faceted.
Comments
Moonstone has been highly valued for centuries, especially by royalty. Currently, its popularity is probably highest in Germany and the Scandinavian nations, where it’s preferred over pearl and alexandrite as the June birthstone. Its characteristic phenomenal effect, adularescence, is named after a mining site at Mt. Adular in Switzerland. (Moonstone is also sometimes called “adularia”).
Adularescence is caused by the diffraction of light as it hits thin, alternating layers of orthoclase and albite within the gem. Generally, moonstones are cabbed with high domes to accentuate this sheen. Those specimens with strong displays often reveal cat’s eyes when cut this way. Asterism is rare in moonstone. When it occurs, the star is four legged.
Varieties
A blue sheen color is produced in orthoclase moonstone if the albite crystals are very fine. (Fine orthoclase and albite plates are dispersed within each other as a result of unmixing on cooling). The sheen is white if the albite plates are thick. The color of the orthoclase may be white, beige, brown, red-brown, greenish, or yellowish. Red coloration is due to goethite (iron oxide) inclusions.
A moonstone variety containing the orthoclase feldspar sanidine is found in Grant County, New Mexico.
Some transparent plagioclase feldspars, such as labradorite, also have thin layers of albite. These produce a blue schiller effect if thin and a white effect if thick. In rare instances, the schiller is multicolored, showing blue with green and/or orange. This phenomenon is called labradorescence. Such stones are known as rainbow moonstones or peristerites. Technically, this is a variety of labradorite, not moonstone. Labradorescence is distinct from adularescence. However, “rainbow moonstone” is generally accepted as a trade name.
Identifying Characteristics
Inclusions
Moonstones are one of the few gems that have inclusions so characteristic that seeing them guarantees their identity. They are characterized by 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.” These inclusions are diagnostic for moonstone.
Moonstones also have rectangular dark areas due to stress cracking or negative crystals. Sometimes a cavity extends from such a rectangular dark area, creating a comma-shaped inclusion.
Myanmar moonstones are characterized by oriented needle inclusions.
Specific Gravity and Refractive Index
Material from Sri Lanka tends to have specific gravity on the low end of the moonstone scale, 2.56. Material from India tends toward the high end, 2.59.
The refractive indices (RI) of most moonstone is usually 1.520 – 1.525 with a birefringence of 0.005. Material from Virginia has RI of 1.518 – 1.524 with a birefringence of 0.006.
Moonstone (Feldspar): India and Sri Lanka (~ 5 each). Photo © Joel E. Arem, PhD, FGA. Used with permission.
Synthetics
Although moonstone has been simulated by milky chalcedony and certain types of synthetic spinel, these substitutes are visibly inferior and easily spotted. Synthetic moonstones haven’t entered the market.
Moonstone gems, left to right: cabbed, carved, and faceted. Photos courtesy of Barbara Smigel, Artistic Colored Stones.
Sources
Myanmar has historically yielded the finest material.
Other notable sources are:
Australia; Austria; Finland; India; Madagascar; Mexico; Norway; Sri Lanka; Tanzania
United States: New Mexico; Virginia
Sizes
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 well cut and very attractive. Moonstone with a blue sheen is the most valuable kind and is rare in stones over 15-20 carats. Stones with a silvery or white adularescence are abundant and available in sizes up to hundreds of carats.
This moonstone was discovered by the first Japanese expedition to Mt. Kilimanjaro in 1918. Said to weigh between 300 and 450 carats, it might be the largest moonstone known. “Moonstone from Africa” by Mr Matthew Hardy Japan is licensed under CC By-SA 3.0
Care
This lovely gem is a popular jewelry stone. However, with a hardness of 6 and a slight tendency to cleave, exercise caution if it’s worn as a ring stone. Protective settings are recommended. Use in brooches and pendants should be safe. In all cases, protect moonstone jewelry from hard knocks. Don’t use mechanical systems like ultrasonic or steam for cleaning. Cleaning with warm, soapy water and a brush is recommended.
Consult our gemstone jewelry cleaning guide for more information.
IGS Newsletter
.jpg){kind=link}
