Traditionally Heat-Treated Sapphires

Mother Nature simply doesn’t create enough fine gem-quality ruby or sapphire to satisfy the demands of consumers. Actually, the problem has been around for centuries, and necessity being the mother of invention, the home cooking of corundum (ruby and sapphire) to improve color and clarity was reported centuries ago.

To satisfy the current demand for natural ruby and sapphire, heating mass quantities of corundum is the only solution. For that reason, the industry has accepted heat treatment. (See “If You Can’t Stand the Heat, Get Out of the Gem Business,” p. 129.)

The practice of heating sapphires and rubies on a grand scale has flourished only recently—since the 1980s—and has made great advancements as technology has improved over the past two decades. Now, it seems, the industry wants to separate sapphires and rubies by how much heat treatment they’ve received, or by the type of heat treatment used.

This is where it gets tricky: first, trying to identify the type of heat treatment, and second, trying to determine the boundaries—i.e., at what point does heat treatment of corundum stop being “traditional?”

History. In the early 1900s, during the Art Deco movement, there was a growing need for calibrated goods to manufacture colored-stone jewelry such as straight-line bracelets and invisibly set brooches. Prior to large-scale heat treating, and with the emergence of crystal growth science, synthetic flame fusion Verneuil ruby and sapphire were considered acceptable substitutes for the real thing. The color of fine natural ruby and sapphire matched that of fine color synthetics. Growing synthetics was an expensive process and an amazing breakthrough. Since both were highly regarded, it isn’t surprising now to find a synthetic or two mixed in with matching natural gems in early 20th-century estate pieces.

Jump ahead to 1979, when gem laboratories began seeing large numbers of heat-treated sapphires. Once Sri Lankans and Thais realized that heat treatment could enhance material they were discarding as scrap, the floodgates opened and natural, heat-treated corundum poured out. What is amazing to most—and alarming to some—is that much of this corundum is of such low quality that the starting material would otherwise be unsalable as is, except for possible industrial use.

(Note: Most recent treatments using corundum-melting temperatures, fluxes, and additional elements yield more saturated colors and more transparent crystals. Synthetic healing also is possible using these processes. Heat treatments like these are not considered “traditional,” since there is much more chemistry involved in this heat-treatment process.)

Color. Heated sapphires come in all colors of the rainbow. This is just one of the many reasons why heat treatment is performed on most corundum. The color is stable, permanent, and throughout the gem, just as it would be in a natural stone. Color follows graining planes and natural color zones. Heating in a reducing atmosphere can brighten, saturate, and even out the color of a gem. Slightly lower temperatures and longer cooling times can help accentuate a star. An oxidizing atmosphere will help lighten the color; in fact, much of the over-dark commercial Australian sapphire is heated in an oxygen atmosphere to lighten it. Higher heat along with quicker cooling times can reduce the visibility of extraneous star-causing rutile needles (“silk”).

“I do not know the percentages, but I believe most of the better sapphires are heated in a reducing atmosphere to dissolve the silk and convert the titanium from the rutile in silk to a coloring element,” says Cara Williams of Bear Essentials, Jefferson City, Mo.

Country of origin. Enhanced sapphires usually are not labeled with a country of origin, since many identifiable inclusions are destroyed in the enhancement process. That said, Williams points out that there’s still a pricing difference if you can tell. “Origin still matters, but it’s difficult for most of us to be certain of the Madagascar/Ceylon origin,” Williams says. “Burma is king, but it doesn’t play a major role in the commercial market as it is too limited.

“Ceylon has the old, established name, but there is not enough difference to make a price difference between it and Madagascar,” she adds. “And with a large percentage of the sapphire sold in Sri Lanka being of Madagascar origin, the market has leveled.” According to Williams, because Ceylon origin still seems slightly more romantic, people are paying more for a Madagascan stone sold in Sri Lanka than for one sold in Madagascar.

Identification of heat treatment. There are only a few visible clues that a retail jeweler/gemologist can use to determine whether their corundum has been heated. The best and easiest clue is the presence of included crystals surrounded by halo-like fractures. These fractures occur when the crystal is heated, expands, and literally breaks the inside of the stone. One other clue: lines of tiny white pinpoint-sized inclusions. These are the remains of what once were rutile needles, dissolved by the heat and recrystallized as small pinpoints. Rapid cooling of the gem prevents the rutile from crystallizing back into needles.

Prices. According to The Guide, prices for sapphire are “based on the assumption of heat treatment.” For fine-quality sapphires of 2-ct. to under 3-ct. sizes, prices for heat-treated blue sapphire range from $550/ct. to $1,700/ct. Extra-fine quality stones are priced at $1,700/ct. to $3,000/ct. Extra-fine golden sapphires range in price from $200/ct. to $350/ct., with the more popular pink sapphire priced at $750/ct. to $1,500/ct. Ruby prices for extra-fine 2-ct. to under 3-ct. stones range from $2,500/ct. to $6,000/ct.

Care and cleaning. Whether it’s natural color or heated, all corundum has a Mohs hardness ranking of nine on a scale of 1 to 10, so your client and bench jeweler don’t have to worry as much about damaging these stones as they do with most other gems. And heated color is stable, even at bench repair temperatures. Still, most wholesalers are cautious: “I always advise removing the stone,” Williams says, adding that she’s never had a customer say they need to replace a stone because the bench jeweler altered the color. “I do give strong warnings, however, never to heat star sapphires, as many have not been heated and have inclusions that cannot take the heat.”

Recommended reading. For more information, see Ruby & Sapphire, by Richard W. Hughes, RWH Publishing, and Gemstone Enhancement, by Kurt Nassau (1994).