Gem Treatment Guide
Guidelines for buying common gemstones
By Arthur Anton Skuratowicz and Julie Nash
Editor’s Note: Enhancing a gemstone is nothing new; recipes for treating emeralds with oil date back centuries. However, buyers of gems or gem-set jewelry do need to be aware of the treatments—both safe and suspect—currently being used, as well as the imitations and synthetics on the market. Only in that way can they (and their customers) be sure about what they’re getting. (The Federal Trade Commission’s “Guides for the Jewelry, Precious Metals, and Pewter Industries” says that it is “unfair or deceptive to fail to disclose that a gemstone has been treated in any manner that is not permanent or that creates special care requirements, and to fail to disclose that the treatment is not permanent.”)
To help buyers make the most informed decisions, the following guidelines have been compiled and updated from Working with Gemstones: A Bench Jeweler’s Guide by Arthur Skuratowicz and Julie Nash (MJSA Press, 2005). If you have doubts about any stone you’re about to purchase, consult a qualified gemologist.
A-C: Alexandrite | Apatite | Aquamarine | Chalcedony
D: Diamond
E-L: Emerald, Feldspar, Garnet, Jade, Lapis Lazuli
M-Q: Morganite, Opal, Pearl, Peridot, Quartz (Amethyst, Citrine, Ametrine)
T: Tanzanite, Topaz, Tourmaline, Turquoise
Alexandrite is not known to be treated, but it is one of the most misidentified stones in jewelry today because of the various imitations and synthetics on the market.
Synthetic alexandrite has the same chemical makeup and crystal structure as natural alexandrite, but it’s made in a laboratory or factor. Prices for synthetic material are a fraction of those for comparable quality natural, but they vary depending on the method used in the material’s manufacture.
Often available in sizes over 1 carat, synthetic alexandrite is very clear and exhibits a strong green/greenish blue to red color change. Microscopic examination by a competent gemologist offers the most reliable means of separating a natural from a synthetic alexandrite.
In our opinion, synthetic color-change sapphire is the stone most often misidentified as alexandrite. Synthetic color-change sapphires are generally worth only a few dollars. They are usually large and tend to have a blue to purple color change (occasionally the change is more greenish blue to purplish red, as shown in the examples at right).
Another alexandrite pretender worth mentioning is andalusite, which does not exhibit color change, but has strong orange and green pleochroism (i.e., it exhibits different colors in different crystal directions) that causes it to be mistaken for alexandrite. Standard gemological tests will clearly separate imitations from natural alexandrite.
Although there are no known enhancements for apatite, we believe that it is possible to fill the material with polymers, such as Opticon, to improve the durability of highly fractured pieces. We want to stress that this is our speculation based on our own experiments, and that we have not encountered this treatment.
Aquamarine comes out of the ground with a yellowish overtone, which gives the stone a light green or sea foam color. The vast majority of aquamarine on the market has been treated to remove the yellow component, resulting in a predominantly blue stone. The treatment is not detectable, and it is considered safe.
Chalcedony may be enhanced by dyeing or heating. Heat treatment is usually performed on dull-gray specimens to turn them into more attractive and marketable orange/carnelian colors. It can be done in a standard kitchen oven and is thus widespread-and undetectable. If you have an orange chalcedony, assume it has been heat treated.
Because it is somewhat porous, chalcedony is very susceptible to dyeing. In fact, most black chalcedony (black onyx) is produced by dyeing a grayish or light-colored chalcedony. The stone can also be dyed emerald green, bright blue, purple, pink, and bright yellow. These colors are very bright, appear unnatural, and, if left in direct sunlight, may fade over time.
Due to the variety of colors possible through dyeing, some types of chalcedony can be misidentified. For example, dyed emerald green chalcedony in vintage pieces can be mistaken for emerald melée when in small sizes. Standard gemological testing using crossed polarized filters will differentiate the two. Also, while dyed blue jasper may look like lapis, it lacks the pyrite inclusions of that gem and tends to take a higher polish.
Chalcedony comprises the following types: agate, bloodstone, carnelian, chrysocolla, chryosprase, dendritic agate, drusy, fire agate, jasper, lace agate, moss agate, onyx, sard, and sardonyx.
Diamonds can be enhanced in several different ways. In our opinion, you absolutely must disclose any and all such enhancements to potential consumers. But it is not enough to simply say, “This diamond is enhanced”; you must enable the consumer to understand how it was enhanced as thoroughly as you do. This approach may seem like an attempt to kill the romance of a sale. However, because diamonds are purchased more than any other gem to commemorate milestones in life, full disclosure is imperative. As appraisers, we can tell you from experience that if anyone buys a fracture-filled stone as “clarity enhanced” without fully understanding that there is a substance other than diamond in their engagement ring, that person will never look at the purchase-or the jeweler-kindly again. On the other hand, a consumer who fully understands the treatment and still purchases the diamond doesn’t have any reason for bad feelings.
Irradiation is one of the more established ways to treat a diamond. Diamonds of low or undesirable color are subjected to irradiation, followed by controlled heating, to achieve bright yellows, blue greens, greenish blues, and very dark greens. Red and pink diamonds occasionally result from radiation treatment as well. If done appropriately, the treatment emits no harmful residual radiation. (While the first attempts at radiation treatment involved radium salts, resulting in olive green diamonds that were highly radioactive, modern treatment carries no such risk.) If in doubt about the origin of a diamond’s color, send it to a laboratory for advanced testing. Spectroscopic analysis can often identify a treated diamond.
Coating the surface of a diamond to change its color has been done for the last two hundred to three hundred years. The first incarnation of this treatment can be seen in antique jewelry, in which colored or silver foil was attached to the back of the stone. This would improve either the color of the diamond or the brilliance of the cut. Modern variations on coatings can range from nail polish or ink marker on the stone’s girdle, producing the appearance of a fancy color, to high-tech thin films similar to those used on camera lenses. Because these treatments are unstable and can be easily removed, they are considered deceptive. If dealing with a fancy-colored diamond, examine the diamond to make sure no such treatment exists. (When examining it, look for iridescence on the surface or a “peeled paint” texture. Also, be suspicious of colors that appear too bright.) In any type of bench work except with a laser, the coating will probably come off and the color will alter significantly. Luckily, coatings on modern pieces of diamond jewelry are quite rate.
Laser drilling is done to improve the apparent clarity of the stone. In this process, a laser is used on the diamond to form a tiny hole that leads to a dark inclusion. Introducing air in this way may be enough to mask the inclusion. If not, bleaching agents can be inserted to lessen its appearance. Laser drill holes are visible with microscopic examination. In the photo at right, four laser-drilled holes and their reflections (at the bottom of the photo) are visible. (Also note the pink and blue flash of fracture filling to the upper right. See below for more about fracture filling.)
A variation on traditional laser drilling uses the laser to cause a fracture that extends from the inclusion to the surface of the stone. The resulting opening, as with the traditional laser drill hole, is used to introduce a bleaching agent. This type of laser drilling is very difficult to detect, since it is designed to appear as a surface-reaching crack (known as a “feather”). Both types of laser drilling are stable when subjected to routine cleaning and bench work.
HPHT is the high pressure/high temperature treatment of a diamond to improve its color. This treatment appears to be permanent and will hold up to cleaning and routine bench work. It is also very difficult to detect, although only a certain type of diamond (Type IIA) accepts the HPHT treatment and the majority of diamonds are not of this type.
Definitive identification of an HPHT diamond needs to be done through advanced testing by a laboratory. Reputable HPHT dealers laser-inscribe the girdle edge of the stone to disclose the treatment. However, these inscriptions can be polished off or inadvertently covered by mountings. Observation of HPHT diamonds in crossed polarized filters sometimes reveals characteristic strain patterns, but researchers are not convinced that these patterns are reliable. Gemologists have also noted that while HPHT-treated stones appear as through they should be very strongly fluorescent in daylight, they show no reaction when exposed to ultraviolet. If you suspect an HPHT-treated diamond and cannot find a laser inscription, send it to a laboratory for advanced testing.
Fracture filling is a treatment in which a glass-like substance fills a surface-reaching inclusion to mask its appearance. It can be achieved only when inclusions reach the surface of the diamond; if they don’t, then laser-drilled holes can be used as conduits for the filling material. Several companies perform this treatment, and their exact methods are proprietary. Fracture filling is often called “clarity enhancement,” although it can also be referred to by the name of the company performing the treatment. Yehuda and Oved are two such eponymous names for these treatments.
Fracture filling can usually be detected with microscopic examination. The filling materials give colored flash effects in certain microscopic lighting conditions, as shown in the bottom two photos at right. (The top view shows gas bubbles trapped in the filling material.) Detection of this treatment is very important, as it is not considered completely stable to all forms of bench work. Cleaning is usually safe, but torch work can cause the filling material to seep out or become hazy. Manufacturers of fillers are improving their product, though, and while some fillers are said to be immune to torch heat, it is impossible to tell the formula of the filling material. Turn the diamond and look at all sides so as not to miss any filling.
Emeralds commonly feature fractures, and these fractures are routinely filled-with oils in antiquity, and oils or polymers in the present. Unfortunately, it is often difficult, if not impossible, to identify the exact substance used. Some in the emerald trade prefer oils because they are the traditionally accepted substance. But oil is unstable and can discolor over time or leak out of fractures, especially in the presence of detergents or heat. Others in the trade prefer polymers or other high-tech substances that have greater color stability and harden once they fill the fracture. Unless you know the exact substance used in the treatment process, assume that it is oil.
Since fracture filling has been done for as long as anyone can remember, it has not been generally disclosed to the consumer. In recent years, this has created a media furor and blatant misunderstanding. In this day and age of disclosure, fracture filling can be viewed as deceptive when pointed out by anyone other than the original seller. Therefore, ask about fracture filling when purchasing any emerald, and disclose its presence to any customer unless you have laboratory documentation indicating lack of treatment. It is also important, as with many other gem treatments, that customers understand clearly that a particular stone would likely lack any appeal without being fracture filled.
Feldspars are not commonly treated. Occasionally, amazonite may be irradiated to enhance its green-blue color, or it may be waxed to improve luster. The other gem feldspars are basically enhancement-free. Sometimes, moonstones may be backed with a black substance to improve the appearance of the stones’ adularescence. These backings can range from spray paint applied directly to the back of the stone to black paper placed under a stone in a closed-back bezel.
The garnet group is rare in that there are no routine treatments to change color or clarity. The only known exception at this time is demantoid garnet from Russia, which can be subjected to low-temperature heat treatment to improve color.
Jade is routinely coated with wax after polishing; this fills in any tiny gaps between the stone’s individual crystals. An ultrasonic or a steam cleaner can occasionally remove or damage this wax and slightly dull what appeared to be a lustrous stone.
Dyes are also common. A large portion of the jadeite found in nature is very light in color or white to gray, and these stones must be dyed to achieve bright green colors and lavender shades. Most of these dyes are fairly stable, but they can fade with age. To show how convincingly the stone can be dyed, the white jadeite cabochon at right was cut in half, and the left side was soaked in warm, purple fabric dye.
In addition, jadeite can be bleached and impregnated with polymers. This is known as “B” jade. Detection of this treatment is difficult and usually requires advanced laboratory testing. “B” jade looks like very high quality material, but is worth only a fraction of the value of untreated jade.
With the exception of wax coating, nephrite is not routinely treated, as the stone’s low value does not warrant the cost of treatment.
Lapis lazuli is often dyed to improve color. This gem often features white calcite patches, which are colored blue to disguise their presence. This results in a stone that appears to be very high quality. To determine if a stone has been dyed, swab a discreet area of the stone (around a drill hole on a bead for instance) with acetone on a cotton swab. If certain types of dye are present, they will bleed onto the acetone-soaked swab. Because this swab test could noticeably damage the stone, we recommend using specialty swabs with hard, pointed ends; they are available through cosmetic stones as makeup applicators. These odd-looking cotton swabs allow much greater control of the placement of the acetone on the stone and reduce the risk of visible damage.
Acetone doesn’t affect all dyes. In such cases, you must rely on observing the pattern of the dye within the natural calcite veins in the lapis. Dyed calcite will appear as dark bluish-violet veins traveling through the stone.
Lapis is routinely coated with wax after polishing; this fills in any surface pits and gives the stone a higher luster.
Due to its popularity, lapis has many imitators on the market. Chalcedony (particularly jasper) is sometimes dyed blue to imitate lapis. Sodalite is a blue stone usually found in Canada and often misrepresented as “Canadian lapis.” Purpler in color than lapis, sodalite has white veins running through it that can cause many to confuse it with lapis. Both sodalite and dyed jasper chalcedony tend to have a higher polish than lapis, and both lack the pyrite inclusions that give high quality lapis shimmering gold flecks.
Imitation lapis is manufactured in a laboratory setting and cannot be called synthetic because it is not chemically the same as natural lapis. This material is quite convincing: It has the even blue color of lapis with the expected pyrite flecks. However, it registers only about a 3 in hardness on the Mohs scale (a measure of hardness used for gems and minerals, with 10-diamond-being the highest level), and it can be scratched easily. To determine if a lapis is an imitation, rub the stone rapidly on paper. Imitation lapis will have a shiny area where it was rubbed. Do not rub a visible portion of the stone; do this only on an area hidden from view.
Morganite comes out of the ground with an orangey pink or peach color. It is often heat treated to remove any yellowish component, resulting in a pure pink or more purplish pink. (The same thing is done to aquamarine to remove the yellowish component that naturally occurs in its color.) This treatment is considered stable. It is not detectable.
Opals can be enhanced in a few different ways. One method of treatment is to subject the opal to smoke. Due to opal’s porous nature, the carbon in the smoke penetrates the opal’s surface, and causes a light opal to appear dark. If a smoked opal is chipped, the thin layer of dark color can easily be seen.
A similar effect can be obtained through sugar treatment. In this treatment, an opal is placed in a sugary liquid (such as fruit juice) so that the sugar can soak into the surface of the stone. The opal is then carbonized with acid to impart a thin, dark layer of color. This treatment is considered permanent. However, if a treated opal is chipped, the underlying color can be visible.
Pearls, whether natural or cultured, form when an irritant (the nucleus) is lodged inside a mollusk. In a natural pearl, the mollusk then covers the nucleus with layers of nacre (very fine crystals of calcium and aragonite). In a cultured pearl, it deposits alternating layers of nacre and conchiolin, a protein-like substance similar to human nails). Many pearls of light body color have been bleached to improve color matching. Pearls can be dyed to almost any color; bright pinks, purples, greens, yellows, and blues are common. Exotic colors, such as bronze or black, can be achieved through irradiation.
Dyes tend to concentrate around drill holes or blemishes, while irradiation often turns the conchiolin layer of cultured pearls dark. To help identify either of these treatments, look down drill holes under magnification.
In the case of very high-end pearls, coatings have been an issue. These coatings—used to improve the luster of the pearls—are difficult to detect because they are not visible to the naked eye. Since microscopic examination is also not always accurate, use a qualified laboratory if you suspect a pearl has been coated.
Peridot is not routinely treated to improve its appearance, which makes it a good choice for customers concerned about owning enhanced gemstones.
Quartz (Amethyst, Citrine, Ametrine)
The quartz family comprises stones of various colors, particularly three popular and versatile gems: amethyst (the purple variety), citrine (the yellow variety), and ametrine (a combination of amethyst and citrine in the same crystal). While quartz is not routinely treated, the iron atoms in citrine and those in amethyst can be altered by heat and irradiation. Thus, heating an amethyst causes it to turn yellow (or into a citrine) and irradiating a citrine turns it purple (or into an amethyst). In fact, much of the citrine on the market today started out as amethyst and was heat treated. Neither treatment is detectable.
Heat treatment is the most prevalent and stable enhancement used on rubies. In traditional heat treatment, the stones are packed in flux or borax and subjected to varying temperatures in controlled heating atmospheres to improve their color and clarity. Specific types of heat treatment can produce stars in certain rubies that would otherwise not be attractive in their untreated state.
A less common form of color improvement is diffusion treatment, in which color-causing chemicals are melted onto the surface of the ruby. Although this treatment is fairly durable, any chipping or deep abrasions can remove the color layer and result in highly visible damage. A gemologist can provide invaluable help in detecting this enhancement.
Another treatment involves filling cavities in rubies with a glass-like substance, which can melt with exposure to a torch. Cavity filling can also be considered a bit deceptive: Rubies are sold by weight, and some of the filled cavities can be quite large and represent a significant percentage of that weight. However, cavity filling can be a byproduct of the heat treatment process and not necessarily an intentional attempt at subterfuge.
If you suspect a ruby has been glass-filled (for example, if its price is too good to be true), use a loupe or microscope to look for the following characteristics: gas bubbles; wispy, veil-like inclusions; repeated parallel parting (breaks along the stone’s structural planes); and blue color flash. Glass-filled rubies will also have inconsistent surfaces, which may include luster differences, dissolved surface spots, or facet junctions, as well as a large number of surface-reaching fractures.
Sapphires, particularly blue stones, are routinely heat treated to improve color or clarity; this treatment is stable and requires no special handling. It is sometimes done in conjunction with the glass filling of cavities, which can be problematic if the stones are exposed to the jeweler’s torch: The filling can melt or change color and texture. To identify a filled sapphire, look for luster differences on the surface of the stone, and for trapped gas bubbles. This can usually be done with a microscope, although with some stones a loupe can be used. If in doubt, contact a gemologist for an accurate identification. We see glass filling most commonly in pink and purple sapphires.
Blue sapphires are commonly subjected to diffusion treatment. In traditional diffusion treatment, titanium and iron are placed on the surface of the stones, which are then heated. These color-causing chemicals diffuse into the sapphire’s surface, resulting in a thin layer of rich blue color. While this treatment is stable when exposed to a jeweler’s torch, if a treated stone is chipped, abraded, or seriously scratched, the color layer can be removed, leaving a visible colorless area. To detect the treatment, view stones in diffused white light or while they are covered in water, wintergreen oil, or methylene iodide. The most obvious sign of treatment is concentrations of color along the facet junctions. In the photos at right, the top sapphire shows no evidence of traditional diffusion treatment, while the bottom sapphire displays the tell-tale color concentrations.
Another form of diffusion treatment uses beryllium instead of titanium and iron. Also known as bulk or lattice diffusion, it is used to produce the famed pink-orange Padaparascha color from pink sapphire. Nearly any other color of sapphire can also be improved or changed using this treatment. This form of diffusion appears to be stable when exposed to a jeweler’s torch. It is also not seriously affected by chips and abrasions because it extends farther under the surface than traditional diffusion treatment. Because research into this new form of treatment is ongoing, we recommend keeping up with trade magazines to stay current on the research.
Spinel is unique in that there are no known treatments performed on it to enhance color or clarity. This can be a very good selling point for customers who have aversions to treated gemstones.
Synthetic spinel is available and is often used to simulate amethyst, peridot, and emerald. To simulate the birthstones, two colorless pieces of synthetic spinel are glued together using a colored glue. To detect these assembled triplets, look for the separation of the two pieces of synthetic spinel that form the top and bottom of the gem. This separation is visible with magnification.
The vast majority of tanzanite comes out of the ground a yellow-brown color with slight hints of violet. The stones are then heat treated to create the rich blues and purples that have made tanzanite one of the most popular gems on the market today. This treatment is considered permanent and stable.
The most common treatment is the irradiation of colorless or very light yellow topaz to cause a blue color. This process is safe, as any residual radiation quickly dissipates. There is so much blue irradiated topaz available today that the different shades have trade names, such as Swiss blue or London blue, to denote slight variations in hue.
In recent years, some topaz has been diffusion-treated or coated with thin film technology. These treatments can give an iridescent appearance to the stone or change the color altogether, but the coatings can be damaged by steam and ultrasonic cleaners. Blue and green are currently the most common colors, but brown, red, and yellow are also possible. To determine if a stone has been treated, use diffused back lighting to check for flaking of the coating. Some stones can also have what will appear to be a greasy surface. If you are in doubt about a stone, contact a gemologist.
Tourmalines are sometimes treated with heat or irradiation to improve color. The treatment used depends on the color of the gem. Lightly colored stones are commonly irradiated to produce red (or rubellite), which tends to bring higher prices than many of the other colors. This treatment is not detectable, but it should be noted and mentioned to clients as a high probability nonetheless.
Very dark stones may be lightened with heat treatment. As with irradiation, this treatment is undetectable but should be noted and disclosed.
Turquoise is often, if not almost always, enhanced. Most enhancements are geared toward stabilizing the color of the stone. Due to its porosity, turquoise absorbs many substances that it encounters during daily wear. Skin oils, lotions, mineral-rich tap water, and soap are just a few of the substances that can be absorbed by turquoise and change its color. A stone that was purchased in a robin’s egg blue color can turn green or darken from the absorption of various common substances over a few months of wear. To stabilize the color, turquoise is treated with a wide range of substances to reduce its porosity. On the low-tech end of the enhancement spectrum are simple procedures, such as rubbing the stone with melted candle wax or paraffin. Other, more advanced methods include applying resins such as Opticon, which is designed to seal fractures and fill pits in gemstones.
The color of turquoise can also be enhanced through the use of dyes, which turquoise will absorb easily, thanks to its porosity. Treatments can also satisfy the desire for certain veining patterns. For example, black matrix can be imitated simply by applying black shoe polish or paint to uneven surfaces.
Another turquoise treatment is the “Zachery Process.” This proprietary process results in an even, stable color with high luster. The process uses turquoise’s porosity to absorb a substance that actually decreases the porosity, creating a smoother surface, higher polish, and a stabilized color.
Turquoise treatments are sometimes easily identified. Black matrix added to the stone becomes apparent when viewed microscopically. Some coatings are also visible using a microscope, or will sweat when exposed to a heat source. Other treatments can’t be easily identified. The Zachery Process is nearly undetectable and is best addressed by a gemological laboratory. However, definitive identification of a turquoise treatment is often a very low priority because the cost of testing is often greater than the cost of the stone. Many turquoise dealers claim to be able to instinctively tell if a stone has been enhanced. However, when colleagues of ours tested the “experts” against sophisticated chemical analysis, the human instinct for turquoise treatment was shown to be a miserable failure.
Another issue with turquoise is that it is often backed with epoxy to add strength to a stone. A thick layer of any kind of epoxy-type substance can be added to the side of a stone before or after the gem is shaped and polished, and the treatment is visibly obvious. The exception is if a stone is set in a closed-back setting that covers the epoxy.
When dealing with turquoise, you should assume it’s been treated if you aren’t sure.
You can also find imitation and synthetic turquoise on the market. Common imitators include dyed blue howlite, although it is usually very dull and has a too-bright blue color. Synthetic (laboratory-produced) turquoise is made in plain blue and with matrix. It has a very even tone and will take a high polish. Microscopic examination by a gemologist will reveal structural characteristics that separate synthetic from natural turquoise.
Other turquoise-inspired products are reconstituted turquoise, plastic, and chip inlay. The most common version of reconstituted turquoise is ground turquoise reformed with binders and other compounds into a solid material. Plastic is self-explanatory, and chip inlay consists of small chips of turquoise suspended in clear epoxy.
Arthur Anton Skuratowicz and Julie Nash together operate Anton Nash, an appraisal company based in Colorado Springs, Colorado. Arthur is also founder and president of the Jewelry Training Center, also in Colorado Springs.