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Star Sapphire

Blessed is the corundum that contains rutile. Not only does this mineral produce asterism, the star-effect, in sapphires, it also acts as a bluing agent when combined with iron. There’s only one drawback.

Rutile (titanium oxide) 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, 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 into star stones was the fate of many rutile-rich but otherwise useless corundums for centuries. Then about 25 years ago, dealers in Bangkok discovered the effects of dissolving rutile in ovens. Altered-state rutile turned cloudy stones clear and bland stones permanently blue.

This alchemical breakthrough sent Bangkok cutters scurrying to Sri Lanka to buy up heat-transformable stones by the ton 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 sow’s ears were far from cheap. As Sri Lanka’s mountainous backlogs of Geuda material became molehills, 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 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 star stones are earmarked for oven burning in Bangkok. But long before Thai cooking endangered this gem species, star sapphire suffered a foretaste 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 Union Carbide debuted the first synthetic star sapphires, trademarked as Linde Stars, in the late 1940s. The new manmades boasted stars with sharp, straight rays that made most of the natural variety’s best seem crooked and blurry by comparison. No matter that the perfection of the Linde star offended, and still offends, traditionalists. 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-intensity blue color connoisseurs associate with this species.

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Star Sapphire