Treatments for colored gemstones are any processes that are performed on the stones other than cutting and polishing. Various treatments can alter the color, clarity, durability, and ultimately the value of the colored gemstone. These treatments can run from accepted, everyday practices to deceptive manipulations that can greatly devalue the stone.
By far the most common form of treatment for sapphires, rubies, tourmaline, and spinel is heat treatment, though there are others, and the differences between them can mean a tremendous variation in the value of the colored gemstone.
Colored Gemstone Treatments for Rubies
Sapphires and rubies are both comprised of the mineral corundum, though the red variety, ruby, is considered a separate colored gemstone. Rubies in their natural state often feature tiny, needle-like rutile inclusions that can sometimes cause a polished ruby to exhibit ‘asterism’—a highly prized ‘star’ effect. But these same rutile inclusions are often accompanied by a decrease in clarity, and this compromised clarity can devalue the colored gemstone.
Heat treatment can reduce the amount of inclusions and greatly improve the overall clarity and color of rubies. A practice that dates back hundreds of years, heat treatment is a relatively simple process, and is considered acceptable in the industry as long as it is disclosed.
While heat treatment can increase the value of a given colored gemstone by improving its clarity and color, untreated rubies that are naturally clear with good color always command much higher prices. A nice looking five carat ruby that has been heat treated might be valued at $2,500 per carat, but a nice looking untreated five carat ruby might fetch as much as $10,000 per carat or more.
With such a large difference in value, it is of course extremely important that any heat treatment be disclosed. Detection of heat treatment in rubies is possible because the treatments modify natural inclusions. Gas or fluid inclusions can be destroyed by heat treatment, and for rubies with rutile inclusions, the margins of the needles can become diffuse. These features are usually detectable by a gemologist using a microscope.
The absence of evidence of heat treatment can suggest an untreated stone, though verifying that a ruby has not been heat treated is a bit more difficult. Any untreated colored gemstone should be accompanied by a gemstone grading report from the GIA (Gemological Institute of America) or the AGL (American Gemological Laboratory).
Other treatments sometimes used for rubies include fracture filling and flux healing. In these processes, the ruby is heated to high temperatures in the presence of a chemical flux, which melts and penetrates surface reaching fissures. When cooled, the flux either induces re-crystallization within the fissure (flux healing), or it solidifies into a glassy mass (fracture filling). While this process can make a low value, poor quality ruby more marketable, it can also make the colored gemstone extremely fragile, and its value compared to a natural or heat treated stone is negligible.
Colored Gemstone Treatments for Sapphires
As sapphires are simply the non-red form of corundum, they are often treated in the same way as rubies, with some notable exceptions. Like rubies, sapphires are often heat treated to improve color and clarity. Treatment under relatively low heat (4000 C) has been commonly used for many years, though recently high temperature treatments (above 17000 C) have resulted in dramatic changes in color. Knowledgeable gemologists should be able to discern if a sapphire has been exposed to this extreme heating, though confirmation from an independent laboratory is the only way to confirm that a sapphire has not been heat treated.
Fracture filling and flux healing are also used on sapphires, with the attendant decrease in the value of the stones. As with rubies, natural, untreated sapphires carry the highest premium, with mildly heat treated stones considered acceptable when the treatment is disclosed, and filled or healed sapphires retaining little value.
Sapphires are also sometimes subjected to beryllium and titanium diffusion treatment. In this process, the sapphires are heated to near the melting point, and the color-inducing elements diffuse into the surface of the stones. With beryllium diffusion, the color only penetrates the surface level, and the original weak color of the stone can be seen if it is nicked or repolished. Diffusion treated sapphires are generally worth less than stones treated with heat alone.
Colored Gemstone Treatments for Spinel and Tourmaline
The same kind of heating used for rubies and sapphires is also used for spinel and tourmaline, though not nearly so widely. Possessing one of the widest color ranges of any colored gem, tourmaline is sometimes heated to lighten a deeply saturated color. The resulting colored gemstone is stable, and the process, when disclosed, is accepted in the industry.
Other treatments include irradiation with Cobalt-60 to intensify color, and fracture filling. Both these treatments negatively affect the value of the colored gemstone, with any filled tourmaline rendered nearly worthless.
Spinel is rarely heat treated. Until quite recently, it was thought that heating did not improve spinel, though recently some spinels from Tanzania have been heat treated to improve their transparency. In addition, some “off-color” natural spinel has undergone a high temperature diffusion process with Cobalt to induce a strong blue color, though the practice is not widespread.
Where to Sell a Valuable Colored Gemstone
If you are looking to sell estate jewelry set with high quality rubies, sapphires, tourmaline, or spinel, contact Diamond Estate Jewelry Buyers today. We are widely recognized as the best place to sell expensive gemstones in the country. Our jewelry buyers specialize in antique colored gemstone jewelry, as well as estate jewelry from Tiffany & Co., Cartier, Harry Winston, Van Cleef & Arpels, and other luxury brands.