GO 340 Gemstones & Gemology
Heat treatment may change crystal inclusions within the gem, causing them to melt or explode. This may be detected with magnification by a skilled person, although it may be difficult to definitively state any color is natural when the gem material is flawless. Temperatures used for heat treatments vary, depending on the material and desired color. Sometimes low temperature, such as that from an alcohol lamp, will change brown topaz to pink; very high temperatures, as high as 2050 degrees C, are needed for other alterations, such as titanium-rich milky sapphires to blue. Amber Beryl Corundum Quartz Topaz Zircon Tanzanite Tanzanite, an important gem variety of zoisite, is strongly pleochroic, exhibiting violet, blue, and yellow to green. The yellow-green component is removed with heating, resulting in the blue or purple final color. It is assumed that all tanzanite is heat treated. Radiation is energy emitted in the form of particles or electromagnetic rays. Ionizing radiation creates crystal structure defects, which can take colorless beryl and turn it to golden beryl or heliodor and intensify the pink or red in tourmaline. Intense yellow or orange colored sapphire is irradiation induced, but the color is not stable. "The first documented artificially irradiated gemstone was diamond, in which a green color was induced by burying the stone in radium salts" (Hurlbut and Kammerling, 1991, p.170). Unfortunately this produced residual radioactivity, making the stone too radioactive to be safe. Neutron and electron irradiation are preferred methods today for coloring diamonds. It may be very difficult to diagnose irradiation vs. natural color in diamond with the exception of blue. Natural blue diamonds are colored by boron and are electrical semiconductors, while irradiated blue diamonds are electrical insulators. Irradiation is also used on quartz for a smoky brown to black color. Pink spodumene can be irradiated to produce the green variety, known as hiddenite, but it is not a stable color. Blue topaz is the most commercially produced irradiated gemstone in today's market. Natural blue topaz is pale but radiated material creates a deep blue, referred to as Electra Blue, Swiss Blue, and Max Blue, among other names. Irradiating topaz may produce a secondary yellow to brown color that is converted to blue with heat treatments. "Linear accelerator (linac) treatment is a preferred enhancement method for topaz today (Hurlbut and Kammerling, 1991, p. 171). Darker blues are attained, called sky blues, and the process must be followed by heating. The "London Blue" coloration is created using irradiation from nuclear research reactors, which produces residual radioactivity causing the material to be stored until the induced radioactivity decays to acceptable levels. Emerald Diamonds Other Material Colorless coatings include waxes, paraffin, and plastics. To detect coatings, a hot needle may cause wax and paraffin to liquefy and flow, whereas plastics will have an acrid odor. Colored impregnations have been employed to change white opal into black opal and to change the colors of marble and soapstone. Emerald and Ruby Quartz Chrysoprase Jadeite and Nephrite Lapis Lazuli Other Gems Photo date 10/99; © by S.W. Aber Visit the American Gem Trade Association Gemstone Manual, http://www.cigem.ca/368.html.
This page originates from the Earth Science department for the use and benefit of students enrolled at Emporia State University. For more information contact the course instructor, S. W. Aber, e-mail: saber@emporia.edu Thanks for visiting! Webpage created: 1999; last update: March 2, 2008.
Heating Smoking Diffusion Irradiation Fracture and
Cavity Fillings Colorless Coatings
Impregnations Colored Coatings
Impregnations Dyeing Bleaching Laser Drilling Heating
Heat altered gem material is changes or improves the color. Some heat treatment is permanent and can lighten, darken, or completely change the color of the gem. Some heat treatment is unstable and can revert to the original pretreated color with time. Zircon can be unstable and after heat treatment the stones can be exposed to sunlight for several days and then stored in the dark up to a year to remove the unstable stones (Hurlbut and Kammerling, 1991, p. 169).
Amber is heated to change water bubbles to discoid fractures (disk-like or radiating), known as sun spangles; heating can also change lighter yellow amber to darker reddish amber. Cloudy amber, with tiny gas bubbles, may be clarified with heating while it is immersed in an oil (e.g., rape seed or linseed oil).
Heating aquamarine, blue-green variety of beryl, will remove yellow and turn the stone to a more desirable blue; this same treatment is done with morganite, turning the stone from peach to a pink beryl. "In both these cases it is believed that the heating converts yellow-color-producing Fe3+ to Fe2+, the latter having no effect on body color when it occurs in the structural sites in which the Fe3+ produces the yellow coloration" (Hurlbut and Kammerling, 1991, p. 166). This process is not easy to detect, nor is the heating of yellow to light brown chalcedony (which contains iron) to produce red carnelian (converting limonite to hematite).
"The heat treatment of corundum is one of the most widespread and commercially significant of gemstone enhancements. It is generally believed that the vast majority, if not all, of the blue sapphires and rubies seen in the jewelry trade today have been subjected to one or more high-temperature heatings" (Hurlbut and Kammerling, 1991, p. 166). Heating is done to induce or intensify the yellow in golden sapphire; these treated sapphires lack the typical, strong orange fluorescence (long-wave UV) of untreated yellow sapphires. Heat treated blue sapphires could be detected by presence of discoid fractures, patchy color zoning, or a chalky greenish fluorescence (short-wave UV). Heating titanium-rich corundum and cooling slowly may result in acicular rutile to exsolve to create asterism.
Heating quartz is common, to lighten purple and brown coloration (reversing the radiation-induced crystal structural damage), or to produce citrine (yellow to orange quartz). Bi-colored quartz can result, termed ametrine, with both purple and yellow quartz. Heating purple quartz can create green, marketed as prasiolite. Heating golden tiger eye can produce a red variety (dehydrating the limonite to produce hematite).
Brown to orange topaz is colored in part because of chromium, and also because of crystal structure damage. Heating this topaz repairs the structural damage, reducing the yellow component, and turning the brown to orange topaz pink. The material has stronger dichroism than untreated pink topaz. Topaz that is irradiated produces a crystal structural damage, creating a yellow and blue color; heating follows irradiation, reducing the yellow component, and leaving blue as a final color.
Reddish-brown zircons can be heated to 900-1000 degrees C, in a reducing atmosphere, to produce blue, colorless, or some undesirable color. The undesirables are then heated in an oxidizing environment, converting them to colorless or yellow, red, or orange colors.
Smoking
Smoking is a technique used exclusively on opal. Opal is wrapped in brown paper and charred, which causes a thin dark brown coating that intensifies the fire or play-of-color. When the coating wears off, the black opal appears brown. It is easily detected with wetting the gem. Whereas natural opals show the same fire wet or dry, the smoked opal's fire diminishes when wet but returns when dry.
Diffusion
Diffusion treatment is a process which alters the color by exposing the surface to certain chemicals and heating. It has only been successful with corundum, especially with blue sapphire. Faceted stones that did not respond to heat treatment alone, are coated with a slurry of aluminum oxide plus iron and/or titanium (if want blue), chromium oxide (if want red or pink), nickel compound (if want yellow). The stones are heated to temperatures that approach melting and the color-causing agents diffuse into the stones, creating a thin layer of color (Hurlbut and Kammerling, 1991, p. 169). The color is confined to the surface and does not penetrate throughout the gem, which could present a problem if the gem was chipped and needed to be recut (Matlins and Bonanno, 1998, p. 126).
Radiation
Artificial irradiation is the most controversial process used to alter a gems appearance and many times the colors are not stable in light or low heat. Health risk is a concern, as there are still questions about the acceptable levels of radioactivity a gem can carry. The Nuclear Regulatory Agency is currently working on establishing standards. "Commercially three types of facilities are used to treat gemstones: gamma ray facilities (often using cobalt-60), linear accelerators (producing high-energy electrons), and nuclear reactors (producing high-energy neutrons) (Hurlbut and Kammerling, 1991, p. 170). The GIA Gem Trade Laboratory can test gems and grade for acceptable or unacceptable radiation levels (Matlins and Bonanno, 1998, p. 126).
Fracture and Cavity Filling
Filling fractures and cavities with a substance having a refractive index closer to that of the material (as opposed to air), makes breaks less noticeable, which improves transparency and/or clarity but not color. Fracture filling can be colored but this is considered under dyeing.
Emerald has the longest history of fracture filling, due to its popularity and its tendency to be highly included and fractured. Natural oils have traditionally been used for fillings, such as Canada balsam, cedarwood oil, mineral oil, cooking oil, and even motor oil! Cleaning the stone and heat can remove these oils. Recently synthetic resins have been used, such as Opticon, which is more permanent than the natural oils. Treated surfaces are best detected with magnification, in reflected light; dark-field illumination is best for internal break fillings. A flash effect, blue (indicates epoxy resin), orange-yellow (probably epoxy resin), or yellow (sometimes the residue left after the filling has come out), can confirm the presence of resin. Flattened gas bubbles can be trapped in the filling material, slight colored outline of the fracture, and/or areas of low relief can be clues to fracture filling.
Fracture filling, or clarity enhanced diamond, effects the clarity grading of diamonds and is a concern in the trade. The process was begun in the 1980s and is a method of filling cracks with a glass-like substance to improve the overall appearance. The filling material is stable with routine cleaning, but not at temperatures and conditions needed for jewelry repair. The fillings might up the clarity grade but have been slightly yellow, lowering the color rating. Some of the time laser drill holes were made to reach an internal fracture in order to fill it, or introducing a fracture that was not originally there! Detection of fracture fillings in diamond include: an orange flash or blue or green flash interference effect with dark-field illumination; a melted or flow structure in filled breaks; flattened trapped gas bubbles in the filling material (fingerprint pattern); crackled texture in the filling resembling cracks on a dry riverbed (Hurlbut and Kammerling, 1991, p. 173-4).
Opal can dehydrate producing surface crazing. These breaks can be concealed with oil or wax. Chatoyant tourmaline has parallel tubes creating the phenomenon, that can fill with debris from the fashioning process or from wear. The stone can be cleaned with acid and then tubes filled with wax or Opticon resin. Another filling enhancement introduced in the 1980s was filling cavities and pits on the surface of ruby, sapphire, and emerald. These fillings were not oils or waxes, but a glassy material that served to conceal the cavity and also add weight to the reported caratage of the stone.
Colorless Coatings and Impregnations
The purpose of coatings is to protect dye treatments, to improve the polish by masking small scratches, grainy textures, or surface irregularities, and to stabilize porous gemstones (Hurlbut and Kammerling, 1991, p. 174-5). These treatments are used on gem material composed of more than one mineral, such as jadeite, nephrite, or lapis lazuli, to aid in polishing. Aggregate gem surfaces may be uneven and vary in hardness. Gems coated because of low hardness include alabaster, marble, rhodochrosite, soapstone, turquoise, serpentine, and amazonite feldspar. Besides low hardness, some gems are porous and the coatings keep the surface from accumulating skin oils and dirt.
Colored Coatings and Impregnations
Colored surface coatings usually add a superficial color layer that does not penetrate the gem's surface. This enhancement can be detected with magnification if scratches, pits, or nicks appear in the coating. Some blue or purple substances have been used to treat yellowish tinted diamonds to make the stone appear more colorless. The color is usually applied to the pavilion, just below the girdle, a kind of treatment like the material used to coat or tint optical lenses. Another surface coating applied to quartz crystals is a thin layer of gold, which creates a greenish blue color with iridescence.
Dyeing
Dyeing is a treatment that alters the body color of a gem and has been done for thousands of years. For the dye to penetrate, fractures must exist. If the gem is not porous or fractured naturally, the opening for the dye to enter the stone is produced by "quench crackling," a heat-induced thermal shock, that creates a network of fractures (Hurlbut and Kammerling, 1991, p. 175). The stability of dyed gems is dependent upon the type of dye, which varies from natural organic material to synthetic or precipitations of metallic salts.
Emerald and ruby is dyed using a colored oil, which fills in fractures and enhances the depth of color. To detect this enhancement, examine the stone in diffused transmitted lighting and look for color concentrated around fractures. Some green colored oils will fluoresce a greenish yellow.
Colorless quartz can be quench crackled and placed in the dye simultaneously or after drying. Magnification can show the result of the quench crackling. Chalcedony, a cryptocrystalline quartz, has many varieties including agate, onyx, carnelian, chrysoprase, and pseudomorphs after bone and wood. The stone is simply soaked in a solution for penetration, then soaked in another solution to arrive at the desired color.
is a natural green chalcedony colored by nickel, whereas the solution to dye chalcedony green has chromium oxide. This can be detected by spectroscopy or using the color filter (chromium colored will be red and nickel colored will remain green). Blue chalcedony is dyed with cobalt and again can be detected with the color filter, which will show red. Blackening is a technique using a sugar-acid chemical reaction that produces carbon to blacken the color (Matlins and Bonanno, 1997, p. 208). The method is to soak the stone in a sugar solution, then in concentrated sulfuric acid. This treatment produces black opal and dyed black chalcedony, sold as black onyx. This treatment cannot presently be detected but because natural gem-quality black chalcedony is extremely rare, this dye treatment is the norm (Hurlbut and Kammerling, 1991, p. 177). Jasper may be dyed blue to resemble lapis lazuli.
Green and lavender jadeite is routinely enhanced with dying inferior material. Green enhanced jadeite can be detected with spectroscopy. Lavender jadeite, created by dying white jadeite, has no conclusive tests to detect the enhancement although some fluoresces a strong orange with long-wave UV radiation. Nephrite has a more compact texture and is not dyed as often as jadeite.
Lapis lazuli is an aggregate of minerals which include white calcite and pyrite. The white calcite can take a dye to create a more uniform blue. Some dyes can be detected by rubbing the gem with an acetone-dipped cotton swab, unless the gem has been surface coated after dying.
Alabaster, coral, banded calcite, marble, and magnesite are dyed to enhance their color or to imitate. Howlite, a hydrous calcium borosilicate, is a white mineral frequently found with black veins that is dyed to imitate turquoise as seen below. 
Bleaching
Bleaching is used to lighten or remove color and is done with chlorine compounds or concentrated hydrogen peroxide (Hurlbut and Kammerling, 1991, p. 179). This enhancement is done to pearls, black coral, and chatoyant tiger's eye (in an effort to imitate cat's eye chrysoberyl).
Laser Drilling
Laser drilling is used to remove dark inclusions primarily from diamonds. If the heat does not vaporize the inclusion, the hole is flushed with hydrofluoric acid. These holes may appear as whitish channels or as light flashes if a high refractive index material is used to fill the cavity.
The material for this section came primarily from:
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