Table of Contents:
- Standard Gemstone Treatments for Altering Color
- Dyes and Coatings
- Detecting Dyes and Coatings
- Heating and Irradiation
- Dyes and Coatings
- Beryllium Treatment
- “Red Andesine”
- Gemstone Treatments for Clarity
Standard Gemstone Treatments for Altering Color
The oldest method for enhancing the color of gems is the use of foil backings. Modern methods are much more subtle and difficult to detect.
Dyes and Coatings
Occasionally, diamonds are “painted” with a pale-colored dye to offset a yellowish tinge. However, the coating soon wears off.
Quartz is sometimes stained or dyed to resemble jade or tourmaline. Since chalcedony is porous, it readily absorbs dyes to produce a variety of brightly colored stones. “Black onyx” is made by soaking grayish colored chalcedony in a sugar solution and then blackening in sulfuric acid. This process leaves very tiny particles of carbon in the pore spaces of the chalcedony.
Turquoise can receive color enhancements in many ways. Techniques such as soaking in wax (paraffin) or impregnating with plastics are often referred to as “stabilization.” Although you can usually detect such methods, suspect them whenever a deep-color turquoise comes with a modest price.
Grayish jadeite can be stained to produce an “Imperial” color or dyed an intense mauve. Natural mauve jadeite does exist, but the dyed material has a much darker hue. Serpentine, a material not related to jade, can also receive a rich, green stain that creates a color that resembles Imperial jade. (You can learn more about jade treatments here).
Dyes can give coral a more desirable (typically red) color. Black coral can be bleached to produce a “golden” material.
Lapis lazuli is frequently dyed blue or “touched up” with colored solutions.
Detecting Dyes and Coatings
Often, you can readily detect dyes because they tend to concentrate along fine cracks and fissures. You can see these dark lines under magnification. Surface coatings (wax, polymers) are easily scratched. Modern devices, such as Raman spectrometers, offer a high-tech solution for detecting impregnations and coatings. However, even simple tests, such as applying solvents (alcohol, acetone) with a cotton swab and seeing if any color comes off, can sometimes quickly and effectively expose dye treatments.
Heating and Irradiation
Topaz, for example, occurs in various colors. Pale blue topaz isn’t uncommon, but deep, intense blue stones don’t occur in nature. However, gamma irradiation of certain colorless topaz can produce such gems. This treatment turns the material greenish brown, but then heating produces a rich blue color. Some golden or yellow topaz can be heated to yield a pink or purplish red color.
The color of pale brown or “sherry” topaz can sometimes be improved by gamma irradiation, but heating or exposure to sunlight usually reverses the process. Color fading of natural brown, sherry, and some blue topaz isn’t uncommon.
Currently, in the case of blue topaz, there’s no way to detect color enhancement by gamma irradiation. Although some fading may occur, beyond a certain point, the color seems to be stable. The inability to prove (or disprove) treatment destroyed the large premium in value once attributed to naturally occurring blue topaz. This sobering experience should also serve as a warning about the possible future repercussions of non-detectable treatments.
Heating usually lightens the color of tourmaline but can also sometimes turn a dark green stone an attractive, emerald-like color. Gamma irradiation of tourmaline produces spectacular color changes. Pale pink and some colorless stones may turn dark pink. Medium pink material may turn yellow, and blue gems may turn purple. Gamma rays can turn pale yellow tourmalines a peach color.
Heating is standard procedure with zircon, turning the drab and unappealing brown material into lovely and desirable colorless and blue gemstones.
Tanzanite coloration was once a subject of intense study. Heating certain crystals, which eliminates the red-violet color component, produces the lively sapphire-blue color that has made the gem so popular.
Dark blue aquamarine is quite rare and very costly. Many of the dark stones seen in jewelry are produced by heating greenish or brownish material to a temperature of 400-450°C. The treatment results in a permanent color change.
A rare kind of beryl known as “Maxixe type” has a distinctive indigo or cobalt-blue color produced by radiation, either naturally or in a lab. Sunlight and heating both bleach the color of this material to yellow or tan and eventually turn it colorless.
Spodumene isn’t normally heated, although some yellowish brown material may be changed to a purplish color by heating. Lilac kunzite can turn an intense emerald-like green by gamma irradiation.
One of the most commonly heated gems is quartz.
Gamma irradiation plus heating of some Brazilian quartz produces a bright, greenish yellow color not found in nature. This color fades substantially in sunlight.
Although amethyst may turn brownish or red at a temperature between 400 and 500° C, sometimes it turns a green color. While such heated gems are sometimes called “greened amethyst” or “green amethyst,” these are misnomers. Green quartz is properly known as prasiolite. Further heating of these gems causes a complete loss of color. Irradiation plus heating may also produce brown, orange, and yellow hues in quartz.
The heating of amethyst to a brownish yellow color occurs on a commercial scale. The resulting material is often sold as “Madeira topaz.” A lighter shade has been called “Palmyra topaz,” and reddish stones “Spanish topaz.” However, these are misleading names that should be abandoned. (Quartz and topaz are distinct gem species with different properties and values).
The color change of amethyst due to heating isn’t always predictable, and fading is a possibility.
Gamma irradiation of some pearls produces a gray or blueish gray color (though not the “black” found in nature). The treatment can also improve the color of greenish pearls. The gamma-induced color is uniform and doesn’t fade noticeably in sunlight. (Learn more about pearl treatments here).
Heating is an ancient and widely used process for improving gemstone color. With disclosure, it’s considered acceptable by the trade. On the other hand, diffusion can be challenging to detect and, therefore, offers unscrupulous dealers an opportunity to commit fraud through intentional non-disclosure of the process. This artificial enhancement process often radically changes the appearance of a gemstone. The trade doesn’t regard it as “benign,” in the same manner as just heating.
The diffusion process allows chemical impurities to enter the structure of a gem material on an atomic scale. These impurities disturb the structure and affect the way the material absorbs light, thereby producing a color. The atomically disturbed areas are known as “color centers.”
The source of coloration in most transparent gemstones, color centers are ubiquitous in mineral crystals of all types. However, laboratory experimentation has revealed chemical processes that allow forced introduction of impurities into various gem materials, creating artificial color centers and producing hues in these gems due entirely to artificial treatment.
Diffusion rates are extremely low in minerals, unless carried out at very high temperatures that would damage or destroy most gems. Exceptions include minerals such as corundum (sapphire) and feldspar that form in nature by crystallizing from a melt. As a result, they can reach high temperatures, nearly to their melting point, during treatment.
The most extensively studied gemstone diffusion treatment involves the migration of the element beryllium into sapphire. The result creates a new color in the material, frequently orange but also blue, yellow, or red. In large stones, you can see the altered color zone as a surface layer through immersion in a refraction liquid. However, in small stones, the beryllium may penetrate through the entire body, so they have no visible color zones.
Beryllium treatments are now routinely detected with spectrometers and other advanced devices. Unfortunately, most sellers within the jewelry industry don’t use this equipment.
The gemstone widely sold as “red andesine,” a material of questionable origin and marketed in prodigious quantities in recent years, presents a more egregious example of undisclosed treatment.
Laboratory testing has confirmed that the red (and green) color seen in this product can be produced by diffusion of copper into pale, yellow-colored feldspar. The copper entirely pervades the structure of the host and the resulting coloration can be quite uniform (when viewed without magnification). The only localities on Earth that provably yield naturally occurring red and green feldspar are the sunstone deposits in Oregon. The total production from all these localities combined is vastly smaller than the immense volume of cut gems marketed as “andesine.”
It has become generally accepted that “red andesine” is a treated gemstone, despite its continued marketing as a natural stone.
Gemstone Treatments for Clarity
Many gem materials contain microscopic (or larger) cracks and fissures that often reach the surface of a cut stone. These fissures are clearly visible because of the large difference in refractive properties of air versus the host mineral. The cracks “disappear” when filled with a substance (such as oil, wax, epoxy, etc) that closely matches the refractive index of the surrounding matrix.
Emeralds are undoubtedly the best-known example of a gemstone that receives this treatment. These gems almost always display a maze of internal imperfections that the trade has kindly and euphemistically dubbed jardin (French for garden). If an emerald is soaked in a green-dyed oil and heated, the oil may penetrate the stone and fill the cracks, effectively “clarifying” it and improving transparency and brilliance.
Oiling of emerald has been done for centuries, typically right at the source. Eventually, the oil evaporates and escapes, so the stone returns to its original, visibly flawed condition. However, it can be re-oiled again and again. Fillers such as epoxy and polymer offer a more permanent fix. Careful testing can generally detect these treatments.
Many other gems receive clarity enhancements with a variety of “fillers.” Even diamonds routinely have dark-colored inclusions “burned out” with lasers. You can easily spot this laser clarity treatment because the intense light beam creates a microscopic “tunnel” through the diamond. (Some diamond laser treatments leave tunnels that are harder to see. You’ll need a microscope to find them).
This edited five-part series of articles, “Understanding Gem Synthetics, Treatments, and Imitations,” is a chapter from Dr. Joel Arem’s forthcoming book, Gems and Jewelry, 3rd Edition. © Joel E. Arem 2011-2013. The International Gem Society (IGS) gratefully thanks Dr. Arem for his contributions to the field of gemology and for allowing us to reproduce this material.
5-Part Series, “Understanding Gem Synthetics, Treatments, and Imitations”