Blessed is the corundum that contains rutile (titanium oxide). Not only does this mineral produce asterism—the star effect—in sapphires, it also acts as a blueing agent when combined with iron. There’s only one drawback.
Rutile can’t perform both feats at once. It has to be in different chemical states to make stars and to make color. If left in its undissolved state where it clusters in dense bundles of microscopic needles, abundant rutile causes corundums to become anything from translucent to opaque. Usually corundums with heavy concentrations of rutile have a milky appearance, which is why it is called “silk” in the trade. Fortunately, stones with partially dissolved rutile have redeeming gray and light to medium-blue colors. When cut into cabochons, these stones frequently reflect light along their domes in a six-rayed-star pattern—the result of corundum’s six-sided crystal structure.
Cabbing is six-star stones was the fate of many rutile-rich—otherwise useless—corundums for centuries. Then around 15 years ago, dealers in Bangkok discovered the effects of dissolving rutile at high temperatures in ovens. After a rutile-turned cloud streaks clear and bland stones blue. This alchemical breakthrough sent Bangkok cutters scurrying to Sri Lanka to buy up heat-transformable corundums by the ton from dealers ignorant of their latent beauty and glad to be rid of them at pennies per carat. By the time Sri Lanka’s gem trade caught on to the Thais’ corundum capers, these tiny silk purses disguised as sows’ ears were known as “Geuda” goods and far from cheap. As Sri Lanka’s mountainous backlogs of “Geuda” material became models, the Thais began to heat many “Geudas” that had been already cut into star sapphires, hoping the rutile in these stones would work the same color and clarity magic. When they found that it did, the hunt was on for treatment-worthy stars.
Consequently, star sapphire has become the first major casualty of modern gem enhancement as large numbers of potential and existing stones are earmarked for oven burning in Bangkok. But long before Thai cooking endangered this gem species, star sapphire suffered a fresh fate of high-tech trauma with the introduction of a U.S.-made synthetic variety.
The Linde Years
As good as nature is at producing stars in sapphires, she is no match for man. That became obvious when Linde Carbid debuted the first synthetic star sapphires, trademarked as Lindy Stars, in the late 1940s. The new manmade boasted stars with sharp, straight rays that made most of the natural variety’s best seem crooked and blurry. Consumers took to the synthetics’ straight-legged star, so much so that four decades later they have come to expect rays in the far more expensive natural variety that walk lines as straight and narrow as those made in the laboratory.
That’s asking a lot of natural star sapphires, especially those with the medium blue color connoisseurs associate with this species. As said before, asterism results when light reflects from large clusters of rutile needles most common to the corundum found in Sri Lanka. The more densely packed and fine those needles, the more precise the star.
Union Carbide and the present major practitioner of its star sapphire manufacturing process, Nakazumi Earth Crystal in Japan, cram rutile fibers far smaller than those found in natural stones together to make a star so intense it looks, in the words of Gemological Institute of America chief gemologist John Koivula, “painted on the stone rather than emanating from within it.” That’s why these almost branded stars are considered a tip-off to laboratory origin.
But despite stars that lack depth and often appear like decals, synthetic stones have clearly put dealers on the defensive—a tribute to the crystal grower’s art. While viewing stones for this story, expert after expert felt the need to apologize for the diffuse, uneven, sometimes stubby rays of the natural variety, especially common with deep-blue colors.
The Curse of Fine Cutting
Since star sapphire is first and foremost a phenomenon stone, the quality of the star logically means more to jewelers and consumers than color. It is hardly surprising that star sapphire specialists like David Cohen, Rafco International Gem Corp., New York, report selling three times as many top grays as top blues. The stars in gray stones are generally better—meaning each ray fully extends to the base of the cabochon.
Fine stars aren’t all that gray stones have going for them. Usually, they are benefited by superior make. Because blue stones tend to be more translucent than gray ones, cutters have to keep more of the original rough to retain asterism and color. Blue stars are generally cut with sagging bellies while grays are cut with flat ones. Jewelers who do not understand that big bottoms help preserve stars and color resent what they think is needless extra weight and expense.
The ubiquity of slim, trim Lindes and nice-make natural grays only makes it harder for dealers to explain why so many deep-blue star sapphires are overweight. Forgetting for a minute that unsightly bottom bulk can be hidden in mountings, the deep blue star sapphire with near-perfect asterism offers consumers one mighty consolation for excess underneath: rarity.
Even with one or two legs missing sections, fine blue stones can command up to $1,000 per carat in 10-carat sizes. If all the legs are distinct and intact, the price can easily jump to $1,500 per carat or more, perhaps as much as $2,000 per carat—if you can find any. Prices drop to between $750 to $1,000 per carat for fine blues in 5-carat sizes, the minimum dealers recommend for this species to look its best.
The steep prices of fine blues make far less expensive gray and powder-blue colors very palatable to jewelers. At up to $200 per carat for top-color 10-carat gray stones with good, well-centered stars, who can blame them? The extra $100 per carat for a more desirable powder blue doesn’t seem to present too much hardship either.
However, prices will almost surely rise if the dollar continues its slide against the Japanese yen. As Cohen points out, “The Japanese are big buyers of phenomenon stones. So whenever the yen strengthens against the dollar, it gives them more of a buying advantage in Thailand and other Far East sapphire cutting centers.”
Please note: this profile was originally published in 1988 in Modern Jeweler’s ‘Gem Profiles: The First 60’, written by David Federman with photographs by Tino Hammid.
The 112.5-carat star sapphire shown in the header image is courtesy of Leo Boyajian, Naples, Fla.
