GemNotes

BUYER’S GUIDE: MOONSTONE FELDSPAR

This is the ninth in a series by Senior Editor Robert Weldon designed to help jewelers buy different types of gems.

What to look for: Moonstone has an aura of mystery &endash; and for good reason. Properly oriented, the best moonstones glow with a shimmering blue light against an otherwise colorless, transparent to semitransparent body color. This glow seemingly floats over the surface, appearing and disappearing as you tilt the gem.

Gemologists call this glow adularescence; geologists call it the “schiller effect.” By either name, it’s what makes moonstone special. Adularescence is caused by the intergrowth between two types of feldspars: orthoclase and albite. The two feldspars have different refractive indexes, and that’s what produces the unusual, reflective optical effect in certain directions of light.

The key to revealing adularescence lies in how the gem is fashioned. Most moonstones are cabochon cut, which maximizes the angles at which you can see adularescence. (Round beads are common for the same reason, but look for beads that are well matched and have the same color of adularescence.)

Recently, however, we’ve seen more and more faceted moonstones. To facet a moonstone, the cutter has to orient the adularescence to parallel the gem’s table facet.

Moonstones come in a variety of qualities, sizes and colors. The best ones contain few or no visible inclusions, exhibit a high degree of discernible bluish adularescence, range from 3 to 12 carats and have a colorless body color. (The body color may be green, yellow, pink, gray or brown and vary from semitransparent to opaque; the adularescence may range from indistinct to distinct.) The best adularescence is perceived as bluish. A more common adularescence is white with no hint of blue.

Another common phenomenon in moonstone is a distinct “cat’s eye” effect rather than a soft, billowing adularescence. This form is beautiful in its own right, but it’s not as popular as moonstone with adularescence.

Could be confused with: Once you’ve seen a few good moonstones, you’ll be able to separate them visually from other types of gems easily.

Certain chalcedonies may emulate moonstone’s body colors, and more rarely may have an optical quality that appears like adularescence. But tilting the gem and examining it from all angles will show that it’s not the same optical effect as true adularescence.

If you’re still unsure, use factors mentioned in the gemology section below to distinguish moonstone.

Enhancements: Moonstone enhancements are rare. Black coatings are sometimes observed on the pavilions in older jewelry. This coating “enhances” the adularescence; generally, it can be scraped off (carefully).

Supply & price: Feldspars are the most common minerals after quartz. But good moonstones are quite uncommon &endash; especially those larger than 5 carats. Most fine moonstones are from Sri Lanka, where they were once discarded in the search for ruby or sapphire. Moonstones are found also in Australia, Brazil, India, Madagascar, Myanmar and Tanzania.

Gemology: Hardness: 6.0-6.5 on the Mohs scale. Refractive Index: 1.518-1.526. Specific gravity: 2.58. Cleavage: distinct. Toughness: poor. Inclusions: a series of minute cracks may appear inside the gems, associated with the intergrowth of the different feldspars. These inclusions often resemble centipedes and are diagnostic of moonstone feldspar.

Care: While moonstones are not affected by light, they are somewhat fragile. A good knock may cleave or part a moonstone. They also are attacked by hydrofluoric acid and have a low tolerance of heat. Lukewarm, soapy water is best for cleaning moonstones.

MORE BLUES FOR THE TOURMALINE BUSINESS

Have you consigned yourself to the idea that Paraíba tourmalines are a thing of the past &endash; a once-in-a-lifetime gemstone occurrence that has come and gone?

It’s true that the source of the electric blue and green Paraíba tourmalines &endash; the Batalha mine in Paraíba, Brazil &endash; is depleted. But there’s been a reprieve with the discovery of a new source of gems with the “Paraíba” look about 32 miles away near Parelhas in the state of Rio Grande do Norte, Brazil (see JCK, October 1995, page 16). The new mine &endash; called Carascal Milionario &endash; is owned and operated by Raniere Addario of Governador Valadares in Brazil.

Gems from the new find may not be exactly like Paraíba tourmalines, but there are some common links. The gems from both deposits produce arrestingly saturated, unique tourmaline colors. And in both cases, the colors are due to the presence of copper.

But there are differences, says Brian C. Cook, a mining geologist who toured the mine and who owns Nature’s Geometry in Grayton, Cal. The Parelhas tourmalines are a homogenous and consistent “sky blue,” he says. The Paraíba tourmalines covered a larger range of colors (sometimes the crystals had different colored cores). And Parelhas tourmalines require no heat treatment. Paraíba tourmalines were sometimes heat-treated.

This color is still considered very rare in tourmaline, so prices aren’t expected to change. (Paraíba tourmalines have been selling for $2,000-$5,000+ per carat.)

Production and sizes have been small, thus far. The monthly yield is about 2.2 pounds of 1- to 3-ct. stones. These will yield calibrated faceted and cabochon stones averaging .20 to .30 ct. The gems are embedded in hardrock matrix of feldspar and quartz.

Cook says that when he visited the mine, he went into a hillside about 260 feet, then down 100 feet on ladders to reach the copper-bearing tourmaline zone. “The pegmatite is fat,” he says, “and I do predict a supply will be available at Tucson in 1996.”