GO 340 Gemstones & Gemology
ES 567 Gemstones of the World
Dr. Susan Ward Aber, Geologist & Gemologist
Emporia State University
Emporia, Kansas USA

http://academic.emporia.edu/abersusa/go340/treat.htm

Gem Enhancement
Treated Stones

Enhancing natural colored gemstones has been going on for hundreds of years. Treatments are frequently applied to enhance the color, although practices are also common to disguise clarity imperfections also. Changes can be temporary or permanent. Read on to find out more about the following treatments:

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).

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.

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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). For more information on diffusion see:

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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).

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.

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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.

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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.

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.

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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.

Colored impregnations have been employed to change white opal into black opal and to change the colors of marble and soapstone.

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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.

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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.

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The material for this section came primarily from:

For more information see

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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: esu.abersusie@gmail.com Thanks for visiting! Webpage created: 1999; last update: September 30, 2012.

Copyright 1999-2012 Susan Ward Aber. All rights reserved.