Custom-carved moonstone by Tom Munsteiner, 6.69 cts, 17.4 x 12.1 mm, Tanzania. © The Gem Trader. Used with permission.
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.
The International Gem Society (IGS) has a list of businesses offering gemstone appraisal services.
Generally, the more transparent and colorless the body and more blue the adularescence, the higher the moonstone value.
Cabochons of translucent material, either white or with pleasing body color and adularescence, are fairly common on the market and command relatively modest prices.
Moonstone cabochons. Photo by Adrian Pingstone. Public Domain.
Large quantities of near-opaque material with various body colors, carved into simple “moon faces” and other figures, are inexpensive and readily available.
On rare occasions, some transparent stones are faceted.
Square cushion-cut moonstone, 5.07 cts, 10.2 mm, Ziller Valley, North Tyrol, Tyrol, Austria. © The Gem Trader. Used with permission.
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.
Star moonstone, 16.08 cts, round cabochon, India. Photo courtesy of liveauctioneers.com and Jasper52.
For more information on moonstone quality factors, consult our buying guide.
| Data | Value |
|---|---|
| Name | Moonstone |
| Is a Variety of | Orthoclase |
| Crystallography | Monoclinic |
| Refractive Index | 1.518-1.525. Typically 1.520-1.525. Virginia material has RI 1.518-1.524. |
| Colors | Body colors range from colorless through shades of yellow, gray, green, pink, reddish, orange, and brown. |
| Luster | Vitreous |
| Hardness | 6-6.5 |
| Wearability | Poor |
| Fracture | Uneven, conchoidal |
| Specific Gravity | 2.56-2.59 |
| Birefringence | 0.005–0.006 |
| Cleavage | Perfect and easy in two directions |
| Heat Sensitivity | No |
| Luminescence | LW inert or blue. SW orange. May fluoresce weak pink to moderate red, LW and SW. White to violet in X-rays. |
| Luminescence Present | Yes |
| Luminescence Type | Fluorescent, UV-Long, UV-Short, X-ray Colors |
| Enhancements | Dark coating on back, enhances adularescence; uncommon; easily scratched; detect with magnification. |
| Typical Treatments | Surface Coating |
| Transparency | Transparent to opaque |
| Absorption Spectrum | Not diagnostic |
| Phenomena | Adularescence, chatoyancy, asterism (rare). |
| Birthstone | June |
| Formula | KAlSi3O8 |
| Pleochroism | Usually none |
| Optics | Biaxial (-) |
| Optic Sign | Biaxial - |
| Etymology | After the lustrous, “moonlight” sheen of the adularescent effect found in these stones. |
| Occurrence | Gravels, pegmatites |
| Inclusions | “Centipedes,” pairs of stress cracks, are diagnostic. |
Moonstone gems, left to right: cabbed, carved, and faceted. Photos courtesy of Barbara Smigel, Artistic Colored Stones.
Moonstones are orthoclase feldspar gemstones with varying compositions, typically with alternating layers of orthoclase and albite.
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.
Sterling silver brooch with inset moonstone cabochons, designed by the Danish jeweler Georg Jensen, 1914. Photo courtesy of liveauctioneers.com and Butterscotch Auction.
Moonstones are sometimes called “adularia” after Mt. Adular in Switzerland, a famous source for these gems. This mountain has also lent its name to moonstone’s characteristic, phenomenal effect.
Moonstone crystal showing adularescence, Mt. Adular, Tessin, Switzerland, 7 x 6.5 cm. Photo by Didier Descouens. Licensed under CC By-SA 3.0.
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.
Moonstone rough and cut set, 4.3 x 4.1 x 3.7 cm (crystal, Tvedestrand, Norway), 2.11 cts, 1.35 cts (gemstones, India). © Rob Lavinsky, www.iRocks.com. Used with permission.
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.
Cat’s eye moonstone, 6.01 cts, oval cabochon, Sri Lanka. Photo courtesy of liveauctioneers.com and Jasper52.
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.
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.”
A cushion-cut feldspar gem or “rainbow moonstone” with multicolored labradorescence, 1.37 cts, 8.4 x 5.8 mm, Madagascar. © The Gem Trader. Used with permission.
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.
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.
Moonstones: India and Sri Lanka (~ 5 each). Photo © Joel E. Arem, PhD, FGA. Used with permission.
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:
Moonstone mine, Meetiyagoda, Sri Lanka. Photo by Wouter Hagens. Public Domain.
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.
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.
Consult our gemstone jewelry cleaning guide for more information.
Vintage ladies poison ring, sterling silver with moonstone. Photo courtesy of liveauctioneers.com and Gallery 63.
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