| Cardinal Gemstones |
| Garnet - Mandarin, Rhodolite, Tsavorite |
Source: Brazil, India, Madagascar, Sri Lanka, Tanzania, Zimbabwe Birthstone: January Red (pyrope) garnet has a hardness of 7 to 7.5 on the Mohs scale. The toughness of Garnet is Fair to Good. Garnets crystallizes in the isometric or cubic crystal system, and have a crystal habit which is primarily dodecahedral and secondarily hombohedral or trapezohedral. Pyrope Garnet has a vitreous luster, Andradite and Uvarovite have an adamantine luster. Pyrope garnet gemstones are often totally free of inclusions. Garnet has a refractive index of 2.168. Garnet Crystal Habit Garnet Color Rhodolite Rhodolite (aka pyrope-almandine garnet) is a variety of garnet with a "raspberry red" purplish-pink hue (above, right) that is a combination of almandite and pyrope garnet, containing trace amounts of aluminum (Al) and magnesium (Mg) as allochromatic coloring agents. Primary sources for Rhodolite are found in Kenya, Mozambique, Sri Lanka (Ceylon), the United States, and Zimbabwe. The name Rhodolite comes from the Greek "rhodon lithos" or "rose stone." Tsavorite Tsavorite is a very rare, recent mineral find, first discovered near Komolo, in the Merelani Hills, 50 km south-east of Arusha in Tanzania. British geologist Campbell Bridges found the first piece of tsavorite, in 1967. Tsavorite (aka tsavolite) is a green grossularite variety of garnet with an intense emerald-green, grass-green, or bluish-green hue (above, left) that is a combination of calcium and aluminum, containing trace amounts of chromium (Cr) and vanadium (V) as allochromatic coloring agents. The primary source for Tsavorite is located in bush-country of south-eastern Kenya and north-eastern Tanzania. The name Tsavorite was coined by Tiffany & Company's president, Henry Platt in 1974; named for Tsavorite's discovery in the Tsavo National Park, Kenya. Tsavorite is reminiscent of emerald but with higher brilliance, fire, durability, and a total lack of inclusions. Mandarin Garnet Mandarin Garnet is another rare, and very recent mineral discovery, having been first identified in 1991. Mandarin Garnet (aka Kunene Spessartines) is a very rare variety of garnet with a distinct "pure orange" saturated hue (above, right) that is a manganese (Mn) aluminium (Al) variety of garnet. Spessartite's orange color is ideochromatic. The primary source for Mandarin Garnet was found in alluvial secondary deposits along the Kunene River between Namibia and Angola, in south-west Africa, but this location appears to be mined out. Other sources for Mandarin Garnet or orange spessartines have been discovered in south-western Nigeria in 1994 and more recently in Madagascar and Sri Lanka. Spessartite specimens from Nigeria have a more reddish-orange hue and stones from Madagascar and Sri Lanka are more of an orange-red with brownish overtones that are considered inferior to a true 'orange' Mandarin garnets from Namibia. The name Mandarin Garnet was coined for Spessartite's deep Mandarin-orange color, and the name "Spessartite" comes from the Spessart mountains in northwestern Bavaria. Rainbow Andradite A rare variety of calcium iron garnet called Rainbow Andradite (aka Rainbow Garnet) exhibits unusual iridescence, reflecting and refracting the full spectrum of light in a 'rainbow' play-of-color. The crystals are an unspectacular orange-brown to greenish-gray, with a lamellar structure causing " thin-film interference" that is responsible for the iridescent quality. Rainbow Andradite was first discovered in the 1930s, at Nevada's Andradite mine. The only current sources for Andradite are the village of Tenkawa in Nara Japan, about 30 km south of Kyoto, and in Sonora, Mexico. Chemical composition: Red Garnet (Pyrope): Mg3Al2[SiO4]3 Rhodolite (Almandine): Fe3Al2(SiO4)3 Tsavorite (Grossular): Ca3Al2(SiO4)3 Mandarin Garnet (Spessartite): Mn3Al2(SiO4)3 Garnet Schist, Garnet, Garnet Crystal
em Enhancement Awareness It is important for the consumer to be aware of the fact
that many varieties of gemstones on the market today have
been treated or "enhanced" in order to improve
their appearance. Additionally, there is a constant battle
between unscrupulous gem traders and testing laboratories
to hide and detect new treatments. Consumers should always require disclosure of any alterations or enhancements of a gemstone before making a purchase. This is especially important if the enhancement to the treated stone is not permanent or if the treated stone will require special care. The Federal Trade Commission (FTC) publishes guidelines for the jewelry trade, stating that consumers must be informed of any gemstone treatments that are not permanent OR that "significantly affects" the value. Most comprehensive gemological reports identify treatment types, and when purchasing a large, expensive stone it is advisable to have a full GIA or AGS report done. Gem Enhancement Classifications The designation for identifying enhancements on a gemological report is: "N" (not enhanced), "E" (normally enhanced), and "T" (nontraditional enhancements). Commonly used methods of enhancement are: Semi-Permanent or Temporary Gem Enhancements * Bleaching (B): Used to lighten and/or whiten gemstones
or pearls Permanent Gem Enhancements * Flux Healing (FH): Heat enhancement used to heal fissures
and fractures Most of the 'temporary' enhancements or treatments are basic and self-explanatory, but most of the 'permanent' gem enhancements involve highly sophisticated techniques that can be difficult to detect. Most treatments leave some kind of telltale signature or fingerprint, but as treatment methods become increasingly subtle, they are more problematic. Some of the more involved treatments and enhancements are listed below. These treatments are permanent, but can have a significant impact on the value of a gem. Flux Healing Flux healing involves the filling of surface cracks, cavities, and/or inclusions by exposing the crystal to a combination of heat and borax or other fluxes and solvents to fill voids with molten low-viscosity flux glass. As the flux mixture fills a fracture via capillary action, the molten mixture dissolves the walls of the fracture until the liquid in the crack becomes saturated with the actual mineral's molten solution. Heat Treatment
High-Temperature High-Pressure (HTHP) High-Temperature (2,000º C) High-Pressure (70,000 atmospheres) treatment or HTHP was developed by General Electric in 1999, to lighten or totally remove a brownish hue in some Type IIa diamonds. Type I diamonds have nitrogen impurities that absorbing some of the blue light spectrum, thereby making the diamond appear yellow, while Type II diamonds have structural defects (aka plastic deformations) created during crystal growth, that can cause a brownish color. High-Temperature High-Pressure treatment can in some cases 'repair' these deformations, whitening the diamond's appearance. Type I diamonds which have nitrogen impurities can also have their color altered using High-Temperature High-Pressure treatment. Using HTHP, a company called Nova Diamond creates fancy colors in vivid hues of yellow and green, bypassing the need for irradiation. Diamonds treated by General Electric (Pegasus Overseas Ltd) to remove coloration have the logo "GE POL" laser-inscribed on the girdle, but laser-inscriptions can be removed by polishing. Detection of non-inscribed HTHP treated diamonds is accomplished by gemological testing laboratories using photoluminescence spectroscopy, 'Fourier Transform Spectroscopy' (FTIR) and 'Raman Spectroscopy' to analyze visible and infrared light absorption looking for telltale absorption lines that would indicate high temperature exposure. Additionally, telltale fingerprints that can be seen under a microscope may include dark cracks around inclusions, internal graining, haze, and partially healed feathers. Irradiation
Residual radioactivity in the stone can be a potential concern. The use of a nuclear reactor can create radioactive isotopes in the stone, necessitating storage for a sufficient amount of time for the decay of any residual radioactivity. Lasering & Laser Drilling The laser-drilling of diamonds to reduce or remove foreign crystal inclusions, or iron-oxide stained fractures, has been preformed for over 20 years. Drilling is accomplished using an infrared laser to bore microscopic holes (0.005" diameter) into a diamond, creating an access channel to the inclusion. The diamond is then immersed into a sulfuric acid solution to dissolve any non-diamond crystals and/or staining. The laser-drilling process is followed by glass in-filling to hide the channel, using a glass material with a refractive index that approximates diamond. Several inclusions can be removed from the same diamond using this technique. Visible signs of laser-drilling and subsequent glass-filling can easily be detected under a microscope, and may include semi-opaque white lines that are straight or slightly wavy in appearance, air bubbles and/or flow-lines within the glass-filled hole, and dark circles where the holes breach the diamond's surface. Lattice Diffusion Lattice diffusion is a treatment process using a combination heat and chemicals to 'diffuse' or deposit an element (beryllium, chromium, vanadium, etc.) from an external source into a gemstone in order to alter or improve its color hue or saturation. This treatment is used primarily on corundum (rubies and sapphires) and is considered an unethical practice. Diffusion is a mechanical process where atoms and/or ions move through solid matter. Within a crystal's lattice structure there are missing atoms called "vacancies" that can be filled with a new element's atoms, thereby changing the chemical composition and color of the mineral. Lattice diffusion can be difficult to detect, and therefor, is considered an unethical practice. The diffusion treatment of ruby and sapphire in-particular is major concern within the gem trade. |
Gemstones & Their Treatment Types Most gemstones, with the notable exception of garnet, have a particular treatment, or series of treatments that are commonly used to increase the marketability of the stone. This list represents common gemstones and the typical treatments they receive. * Alexandrite: None Conclusion There is nothing intrinsically wrong with gem enhancements as long as you are made aware of their use. New treatments are being developed all the time, and gemological testing centers are constantly revising and updating their testing regimes to combat the unidentified enhancement that can slip through the cracks. As consumers, we must rely on the testing laboratory's ability to identify enhancements so that a gem's value can be properly ascertained before a purchase is made. |
Semi-Precious Gemstones | Chrysoberyl
Organic Gems Chrysoberyl Gemstones Chrysoberyl Source: Brazil, India, Myanmar, Tanzania, Zimbabwe, Sri Lanka, USA (Colorado) Chrysoberyl is a yellow to greenish-yellow, transparent
to translucent, semi-precious version of Alexandrite. Chrysoberyl,
like Alexandrite, is a beryllium aluminum oxide. Chrysoberyl
is one of the hardest minerals, falling just below corundum
(ruby or sapphire), and diamond. Chrysoberyl commonly occurs
in granites and granite pegmatites while alexandrite occurs
in mica schists. Chrysoberyl occurs in yellow, greenish-yellow,
green, and brown. Most chrysoberyl is found as an alluvial deposit in river
sands and gravels but can also be found in metamorphic
deposits of marble and corundum. Chrysoberyl Crystal System Chrysoberyl is a very hard material, with a hardness of 8.5 on the Mohs scale. Chrysoberyl crystallizes in the orthorohombic crystal system, forming in slender prisms and usually has a "tabular" crystal habit. Crystal twinning is common. Chrysoberyl has a refractive index of 1.745. Chrysoberyl has distinct cleavage and a vitreous surface luster. Cymophane Cat's Eye Chrysoberyl Chatoyant chrysoberyl is called "Cymophane" or "Cat's
eye", and is usually a translucent yellow. The name "Cymophane" is
derived from the Greek words "wave" and "appearance" (wavy
appearance). The term "cat's eye" usually refers
to chrysoberyl although there are chatoyant varieties of
quartz that are mistakenly referred to as cat's eye. When
cutting a cabochon, gem cutters will center the cat's eye
slit so that it bisects the stone. The primary source for
cat's eye chrysoberyl is the Ural Mountains of Russia,
Sri Lanka (Ceylon), Tanzania and the Marambaia valley mines
near Teófilo Otoni in Minas Gerais, Brazil. Faceted Chrysoberyl The chatoyant effect is produced from microscopic needlelike inclusions of rutile. Chrysoberyl can be facet cut, but the cat's eye variety is usually cut into a cabochon. The largest faceted chrysoberyl crystal in the world is from Russia, weighing 66 carats. Chrysoberyl has been mined for thousands of years in Asia where the stone is believed to ward off the evil eye. Chemical composition: BeAl2O4 |
Garnet refers to a
group of minerals commonly found in metamorphic rock and
in ultramafic igneous rock formations. Garnet is classified
as a Nesosilicate in the Silicate mineral group. There
are six common varieties of garnet that are identified
by their chemical composition and color. They are almandine,
andradite, grossularite (tsavorite), pyrope, spessartite,
and uvarovite.
The name "Garnet" (Grenat French, Granat German,
Granate Spanish) comes from the Latin word for pomegranate,
malum granatum, due to the resemblance of some varieties
of garnets to red pomegranate seeds. Red is the most common
color of Garnet but they can be found in a variety of colors.
Garnet colors (below) include: Golden Andradite, Chrome
Pyrope (red), Mozambique, Rhodolite (magenta), Spessartite
(yellow/orange) and Tsavorite (green). There is also an
extremely rare form of color-change (pleochroism) garnet
(shifting from blue to red) found in Madagascar. Garnet
has been used by man since the late Bronze Age, and its
use as gemstone dates back to ancient Egypt.
Garnet Enhancements Garnets are one of the few gemstones
that are rarely treated or enhanced.
Heat treatment or 'annealing' is done using a combination
of chemicals (beryllium, borax, lead, tantalum) and heat
to permanently alter a stone's color. Heating or "cooking" the
stone is done at temperatures ranging from 450º to 1850º Celsius
for a period of 2 to 12 hours or more. One telltale sign
of heat-treatment is the presence of small fractures or "decrepitation
feather" within the stone that appear around natural
mineral inclusions.
Irradiation treatments involve exposing the stone to electromagnetic
rays (ionizing radiation) or gamma rays (cobalt 60 radiation)
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. Neutron bombardment
and electron bombardment are the two main irradiation methods
practiced today.
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