Gem treatment
Gem treatemt is any human-controlled process that improves the look, durability or value of a gem. It is unfair or deceptive to fail to disclose that a gem has been treated.  
Heat Treatment
Heat treatment is the most common form and famous enhancement. Common gems including Ruby, Sapphire, and Tanzanite, are routinely heat treated to improve color and/or clarity. Heat treatment can lighten, darken, deepen, or change a gems color, it depends on the temperture, rate of heating, duration, pressure, rate of cooling, and atmosphere to combining or not of chemicals (beryllium, borax, lead, tantalum). One telltale sign of heat-treatment is the presence of small fractures or "decrepitation feather" within the stone that appear around natural mineral inclusions.
In fact, we always assume these stones have been heated unless they are certified otherwise or we can prove no heat treatment by careful gemological examination of the gems.
Gems may be oiled to enhance their appearance. Due to their formation process and geological history, virtually all emerald crystals develop characteristic feathers, fissures, and inclusions, sometimes referred to as "jardin."
Colorless oils or resins are used to fill these inclusions, improving the apparent clarity of the stone, also reduces fractures. Oiling treatment is not a permanent but generally accepted. Some requires re-treatment. Special care for oiled gems are repairing and cleaning.
Irradiation treatments involve exposing the stone to electromagnetic rays or gamma rays to release electrons from their normal location, moving them to more desirable color-producing locations. Depending on the mineral to be treated, and the desired color alteration, alpha particles, beta particles, electrons, gamma particles, neutrons will be used in the irradiation process. Irradiation treatment uses a linear accelerator to expose the stone to high-energy electrons, a cyclotron for charged high-energy particles such as protons, or an electron-beam nuclear reactor to expose the stone to high energy neutrons.

Many gems were treated by irradiation such as 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. 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.
Diffusion treatment sometime called surface diffusion is an enhancement method by uses a combination of chemicals and heat. Diffusion is a slow process, use for many days to weeks, chemical commonly used on corundum, added iron and titanium during the gem is heated to near its melting point, the components of the same gem material it pentretes the surface to become part of the gem.
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 or repolish. This layer described as colored coating. Diffused star sapphires or other diffused sapphire is only about 0.1mm depth. Diffused gems should be handled with care.
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. 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.
Foil backing, color tint applied. Common technique before advent of
pavilion faceting to improve brilliance. Not so common today, except in
inexpensive imitations.
Bleaching is used to lighten or remove color and is done with chlorine compounds or concentrated hydrogen peroxide. This enhancement is done to pearls, black coral, and chatoyant tiger's eye.
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.
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.
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. 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.
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.
Not treated
Not treated or enhanced. Some gems, such as peridot, spinel, and garnet, do not require treatment. Exceptional specimens of other gems can always be found, but these usually command much higher prices.

The material for this section came primarily from:

 GO 340 Gemstones & Gemology Emporia State University Susan Ward Aber

Hurlbut, C. S., & Kammerling, R. C. (1991). Gemology. NY: John Wiley & Sons, Inc.

Matlins, A. G., and Bonanno. A. C. (1997). Gem identification made easy. Woodstock, VT: GemStone Press.

Matlins, A. L., and Bonanno, A. C. (1998). Jewelry and gems the buying guide. Woodstock, VT: GemStone Press.

Nassau, K., McClure, S. F., Elen, S., and Shigley, J. E. (Winter, 1997). Synthetic moissanite: A new diamond substitute. Gems and Gemology, vol. XXXIII, p. 260-275.

Schumann, W. (1997). Gemstones of the world. NY: Sterling Publishing Co.


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