Historically gem possession has been reserved for wealthy, royalty, or high religious leaders. It has always been human nature to want what others possess, so imitation gems have been common for some 4,500 years, in the form of glass, plastic, composites, and treated gems. It is not against the law to imitate, as long as the true identification is given. It is only fraud when imitations, natural or synthetic, are passed off as a more valuable gem at an inflated price.
“Is it real?” is a question often posed to jewelers or the knowledgeable gem enthusiast, when people view gems. You do not have to be an expert to answer this question, of course “it” is real! The “real” question should be, is this gem natural or material created in a laboratory? After the inital answer is given, one should ask, “What difference does it make?” Imitations and synthetics simulate the genuine gems and minerals and can be quite beautiful. Part of the definition of a gem is the beauty and this is a subjective attribute. If these created stones and imitations are properly labeled as such, and priced accordingly, they can be an affordable alternative to the “real thing!”
The definition of synthetic is material created in a laboratory using basically the same ingredients found in the natural products (Matlins and Bonanno, 1998, p. 123). Some synthetics have no natural counterpart. Synthetic gems have identical physical, chemical, and optical properties as the natural gem material, for the most part. An exception to this, is in the coloring chemical for some synthetics, which can be different from the natural coloring agent. Even though synthetics may replicate the natural gem, they must be identified and prefaced with “synthetic,” “created,” or some origin indicator.
An imitation is an artificial likeness or copy, which could mean a synthetic material or natural gemstone. Imitations are not exclusively synthetics and not all synthetics are meant to imitate some gem. Some synthetics are marketed as a “gem” in their own right, such as CZ or cubic zirconia (usually advertised without a “synthetic” preface; it does have a counterpart in nature, but it is extremely rare). The term imitation is usually applied to glass and plastic although it can refer to natural minerals too. Golden-colored quartz or citrine has imitated topaz in birthstone rings for so long, many people have a difficult time accepting the natural colors of topaz, which are colorless, pink, pale brown, sherry-colored (reddish-orange), and less commonly “yellow.”
Imitations usually resemble the gemstone in color only and are easy to identify as an imitator. “Ancient Egyptians were the first who feigned gemstones with glass and glaze, because genuine were too expensive and/or too rare” (Schumann, 1997, p. 242). One of the earliest imitations to resemble turquoise, prized by Egyptians, and some 7,000 years ago they constructed a turquoise-colored ceramic substance, termed faience, that was used for beads, amulets, pendants, and rings (Matlins and Bonanno, 1997, p. 227). Also, blue glass gems were found in King Tut’s tomb (Matlins and Bonanno, 1997, p. 227).
Glass is amorphous or a created, inorganic substance which is mixed in a molten form and cooled to a rigid form without crystallizing. There are two main types of glass: crown and flint. Crown glass is made with silica, soda, and lime. It is used for bottles, window and optical glass, and costume jewelry. Flint glass is composed of silica and soda, and lead oxide or other metal oxides replacing the lime. This type of glass has been called strass (or stross) after the Austrian who is credited with its discovery, Joseph Strasser. Flint glass was used to substitute for diamond and because of this, was prohibited in the 18th century by Empress Maria Theresa (Schumann, 1997, p. 242). Glass imitations have been referred to as “paste,” which is from the Italian “pasta” meaning dough … “because the ingredients are mixed wet to assure uniformity of the batch” (Hurlbut and Kammerling, 1991, p. 150).
Stones cut from flint glass resembles the gems they are meant to imitate because lead gives a greater dispersion and higher refractive index. Coloring glass is accomplished using a metallic oxide with a purple color associated with manganese; blue with cobalt; red with selenium or gold; yellow and green with iron; red, green, blue with copper; green with chromium; yellow-green with uranium; and “amber glass” with a combination of manganese and iron, and no amber at all! “The final color of the glass is also affected by such factors as the type of glass used, the oxidizing or reducing conditions used during manufacturing, and annealing after manufacture. Colorless glasses are made by adding decolorizing agents called glassmaker’s soaps; these reduce the greenish tint that otherwise ensues from iron impurities.” (Hurlbut and Kammerling, 1991, p. 151) Many cheap glass imitations are “foiled”, that is the pavilion facets covered with a thin metallic film that acts as a mirror to enhance brilliance and sparkle. Colorless glass can be given a face-up color, or color looking down at the crown, by covering the pavilion with a colored film. A translucent look can be achieved by adding tin oxide.
The Slocum stone, developed by John Slocum, is an interesting imitation of opal. It is glass that has various body colors of white, black, near colorless, or orange (fire opal). The “flashes” of color are produced with metal foils (looks like colored cellophane in transmitted light) that resemble parts of a puzzle (Hurlbut and Kammerling, 1991, p. 153). Opal synthesis succeeded in the US. in 1970 (Schumann, 1997, p. 152). Enhancements can be made by coloring black or matrix opal or impregnating opal with artifical resin (Schumann, 1997, p. 152).
Goldstone is colorless glass with flecks of precipitated copper crystals, which result in the glittery aventurescent phenomenon. Deep blue and green “goldstone” can also be found.
Fire Eye is made of …”parallel, tubular gas bubbles produced by aeration, the same process used to carbonate soft drinks” (Hurlbut and Kammerling, 1991, p. 153). Other glass cat’s eye material is produced by fused optical glass fibers, some with distinct hexagonal cross sections for the “fibers” giving it an overall honeycomb effect. “Still another chatoyant glass, Victoria Stone, is partially devitrified and exhibts a silky texture in the recrystallized areas” (Hurlbut and Kammerling, 1991, p. 153).
Alexandrium is a trade name for glass that changes from pink to violet in incandecent to fluorescent lighting; Tourma-like is a trade name for glass that changes from pinkish orange to gellowish green in incandescent to fluorescent lighting (Hurlbut and Kammerling, 1991, p.153).
Some 300 years ago, hollow glass beads were lined with essence d’orient, an iridescent material from fish scales, and filled with wax (Hurlbut and Kammerling, 1991, p. 153). Translucent white glass beads with coatings of essence d’ orient serve as imitations today. Glass and imitation plastic “pearls” will feel smooth when rubbed lightly against front teeth, while cultured and natural pearls feel gritty (Hurlbut and Kammerling, 1991, p. 153). “Until 1945, Gablonz and Turnau in Czechoslovakia were important centers for the glass-jewelry industry. Then this tradition was taken over by Neugablonz in Allgau, Bavaria.” (Schumann, 1997, p. 242). Porcelain, enamel, resins, and plastics also serve as gem imitators. Plastics are formed by heating and/or molding, and called celluloid (cellulose lastic), bakelite (phenol-formaldehyde), plexiglass or lucite (methyl methacrylate resins), polystyrene and polyvinyl resins. The plastic is constructed of long, chainlike molecules called polymers and have a very low hardness. They are sometimes faceted but usually cut en cabochon to imitate gems such as amber, turquoise, jade, to name a few.
The first gemstones to be synthesized occurred in 1838, although they were only of scientific interest (Schumann, 1997, p. 243). A French chemist, A. V. Verneuil, succeeded in producing gem quality synthetic rubies in 1888, termed a flame fusion process (Schumann, 1997, p. 243). The method melts a powdered aluminum oxide with dye additives, and the molten material forms in a pear-shaped “boule.” Although it has no crystal faces, the crystalline structure is identical to the natural gem. Synthetic blue sapphires were produced by 1910 and sometime later, colorless, yellow, green, and alexandrite-colored sapphires were perfected (Schumann, 1997, p. 243). Star rubies and sapphires were created, by adding rutile to the smelting, in 1947.
Synthetic spinels have been produced since 1910, with the verneuil process, although the chemical composition varies from the natural spinel; synthetic emeralds have been produced since the 1940s (Schumann, 1997, p. 246). Industrial quality, diamond synthesis occurred in Sweden and the United States by 1953-4; gem quality synthetics were perfected in the 1970s. A German chemist, I. Czochralski, developed another synthesis method in 1918, where the boule is drawn out of the smelting after a crystal nucleus has been created. While rotating the boule is continually drawn upward and grows on the underside also. In recent years crystals have been “flux-grown,” a created method that more closely resembles natural crystal growth. These laboratory-grown synthetics are more expensive to produce than other methods, but can still make a good alternative for consumers who are unable to afford natural gemstones (Matlins and Bonanno, 1998, p. 124).
Some synthetic imitations of diamond include: synthetic rutile (also known as titania or diamonite, created in 1948); fabulite (occasionally called diagem, created in 1953), stronntium titanate (SrTiO3); YAG (also called diamonaire, created in 1969), yttrium aluminum garnet (Y3Al4O12); GGG or galliant, a gadolinium gallium garnet, (Gd3Ga5)O12; djevalite, a calcium zirconium oxide (ZrO2/CaO), linobate, a lithium niobate (LiNbO3), cubic zirconia (also known as fianite, phianite, or KSZ), and yttrium zirconium oxide (ZrO2/Y2O3) (Schumann, 1997, p. 242-3, 246). Most recently moissanite, silicon carbide, has become a popular diamond imitation (developed in colorless gem quality in 1996, produced by Cree Research Inc., distributed by C3 Inc.) (Nassau, McClure, Elen, and Shigley, 1997, p. 261).
Doublets, triplets, and foil backs are composite stones, or assembled from two or more components. Although they can be assembled to deceive, some composite tones were devised to overcome low hardness or durability of a gem. Doublets are made of joining two pieces with a colorless cement or fusion. Triplets are two layers of colorless material joined by colored cement that imparts the overall color, or three layers with a colorless cement. Foil backs are made by applying a mirror like back to the stone, foil or metallic paint, to enhance the dispersion and brilliance or produce a star-like effect. Genuine doublet or triplets are composed of the same stone on the top and bottom, such as a light-colored beryl joined by a layer of deep green emerald cement. The genuine assembled stone was composed of two pieces of the same gem and used to imitate that gem (e.g., a beryl triplet may consist of two pieces of colorless beryl joined by a green colored cement and meant to imitate emerale). Semi-genuine doublet or triplet has only one portion genuine (usually the crown) or of the species it imitates, such as an emerald imitation with a colorless beryl crown, quartz pavilion and deep green cementing agent. False doublet or triplet is when one (or more) portion is a natural material but none is the gem it is meant to imitate, such as the garnet-glass doublet meant to imitate a ruby or quartz joined with green cement to imitate emerald.
Garnet and glass doublet was once a commonly encountered composite stone, especially before synthetics became routine. Glass is fused to a slice of garnet (usually almandine). Garnet is usually found in the crown for color and durability. Garnet and glass doublets have been constructed to imitate garnet and diamond. Other doublet imitations of diamond include: foil-backed glass, rhinestones (foil-backed rock crystal quartz), and colorless spinel or corundum with a pavilion of strontium titanate. Corundum doublets, meant to imitate ruby and sapphire, can be natural corundum crown glued to a synthetic corundum. Emerald triplets, meant to imitate emerald, consist of natural colorless beryl, colorless quartz, or colorless synthetic spinel, joined with a green cement.
Opal and ammolite (fossilized shell of ammonites in form of aragonite) are found in thin veins or structures. When used as jewelry, these two gems are commonly found as doublet or triplets to increase its durability and make to most of the rough material recovered. It is common to cement a thin slice of opal or ammolite to a backing. The backing could be a piece of common opal, black glass, or black dyed chalcedony. The triplet would be assembled the same way, except with a convex cap of clear quartz, glass, synthetic spinel, or synthetic sapphire is cemented to the top of the opal or ammolite section.
Diamond and diamond doublets are called “piggy-back” diamonds. Another diamond doublet involves a diamond crown and colorless quartz, synthetic sapphire, synthetic spinel, or glass on the pavilion. Jadeite triplets are constructed of a colorless jadeite hollow cabochon glued to a flat base, with the hollow dome filled with a green jellylike substance (Hurlbut and Kammerling, 1991, p. 162). Imitation cat’s eye could be assembled from a hollow cabochon of synthetic corundum, filled with fibrous ulexite (often called TV stone!), glued to a base of a shallow cabochon of synthetic corundum. The opal imitation could be a cabochon of colorless glass or plastic glued to a base of mother-of-pearl shell.
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. Treatments may involve heating, diffusion, irradiation, fracture and cavity filling, coatings and impregnations, dyeing, bleaching, and laser drilling.
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.
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., rapeseed 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.
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.
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 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).
Artifical 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.
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), orangey-yellow (probably epoxy resin), or yellow (sometimes the residue left after the filling has come out), can confirm the prescence 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.
The purpose of coatings is to protect dye treatements, 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 liquify and flow, whereas platics will have an acrid odor.
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.
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. Chrysoprase is a natural green 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 (Matlines 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. Ths 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 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 chyrsoberyl).
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.