Sapphire

From Wikipedia, the free encyclopedia

Jump to: navigation, search

For other uses, see Sapphire (disambiguation).

Sapphire
The 423-carat (85 g) blue Logan sapphire
General
Category	Oxide mineral
Formula (repeating unit)	aluminium oxide, Al2O3
Identification
Color	Every color except a shade of red – which is called a ruby – or pinkish-orange (the padparadscha)
Crystal habit	massive and granular
Crystal system	Trigonal Symbol (32/m) Space Group: R3c
Cleavage	none
Fracture	conchoidal, splintery
Mohs scale hardness	9.0
Luster	vitreous
Streak	white
Specific gravity	3.95–4.03
Optical properties	Abbe number 72.2
Refractive index	nω=1.768–1.772 nε=1.760–1.763, Birefringence 0.008
Pleochroism	Strong
Melting point	2030–2050 °C
Fusibility	infusible
Solubility	Insoluble
Other characteristics	coefficient of thermal expansion (5.0–6.6)×10−6/K

Sapphire (Greek: σάπφειρος; sappheiros, 'blue stone',^[1] which probably referred instead at the time to lapis lazuli) is a gemstone variety of the mineral corundum, an aluminium oxide (α-Al₂O₃). Trace amounts of other elements such as iron, titanium, chromium, copper, or magnesium can give corundum blue, yellow, purple, orange, or a greenish color. Chromium impurities in corundum yield a pink or red tint, the latter being called a ruby.
Commonly, sapphires are worn in jewelry. Sapphires may be found naturally, by searching through certain sediments (due to their resistance to being eroded compared to softer stones) or rock formations. They also may be manufactured for industrial or decorative purposes in large crystal boules. Because of the remarkable hardness of sapphires, 9 on the Mohs scale (and of aluminium oxide in general), sapphires are used in some non-ornamental applications, including infrared optical components, such as in scientific instruments; high-durability windows; wristwatch crystals and movement bearings; and very thin electronic wafers, which are used as the insulating substrates of very special-purpose solid-state electronics (most of which are integrated circuits).

Contents 1 Natural sapphires 1.1 Blue sapphire 1.2 Fancy color sapphire 1.3 Padparadscha 1.4 Star sapphire 1.5 Color change sapphire 1.6 Source of color 1.7 Treatments 2 Mining 3 Synthetic sapphire 3.1 Transparent and tough 3.2 Common applications 3.3 Use as substrate for semiconducting circuits 4 Historical and cultural references 5 See also 6 References 7 External links

Natural sapphires

An uncut, rough yellow sapphire found at the Spokane Sapphire Mine near Helena, Montana

The sapphire is one of the three gem-varieties of corundum, the other two being ruby – defined as corundum in a shade of red—and padparadscha—a pinkish orange variety. Although blue is their most well-known color, sapphires may also be colorless and they are found in many colors including shades of gray and black.
The cost of natural sapphires varies depending on their color, clarity, size, cut, and overall quality – as well as their geographic origin. Significant sapphire deposits are found in Eastern Australia, Thailand, Sri Lanka, China (Shandong), Madagascar, East Africa, and in North America in a few locations, mostly in Montana.^[2] Sapphire and rubies are often found in the same geographic environment, but one of the gems is usually more abundant in any of the sites.^[3]

Blue sapphire

Teardrop cut blue sapphire

Color in gemstones breaks down into three components: hue, saturation, and tone. Hue is most commonly understood as the "color" of the gemstone. Saturation refers to the vividness or brightness of the hue, and tone is the lightness to darkness of the hue.^[4] Blue sapphire exists in various mixtures of its primary (blue) and secondary hues, various tonal levels (shades) and at various levels of saturation (vividness).
Blue sapphires are evaluated based upon the purity of their primary hue. Purple, violet, and green are the most common secondary hues found in blue sapphires.^[5] Violet and purple can contribute to the overall beauty of the color, while green is considered to be distinctly negative. Blue sapphires with up to 15% violet or purple are generally said to be of fine quality. Blue sapphires with any amount of green as a secondary hue are not considered to be fine quality. Gray is the normal saturation modifier or mask found in blue sapphires. Gray reduces the saturation or brightness of the hue and therefore, has a distinctly negative effect.^[5]
The color of fine blue sapphires may be described as a vivid medium dark violet to purplish blue where the primary blue hue is at least 85% and the secondary hue no more than 15%, without the least admixture of a green secondary hue or a gray mask.^[4]
The 423-carat (85 g) Logan sapphire in the National Museum of Natural History, in Washington, D.C., is one of the largest faceted gem-quality blue sapphires in existence.

Fancy color sapphire

Pink sapphire

Yellow and green sapphires are also commonly found. Pink sapphires deepen in color as the quantity of chromium increases. The deeper the pink color the higher their monetary value, as long as the color is tending toward the red of rubies. In the United States, a minimum color saturation must be met to be called a ruby, otherwise the stone will be called a pink sapphire.^[6]
Sapphires also occur in shades of orange and brown. Colorless sapphires are sometimes used as diamond substitutes in jewelry. Natural padparadscha (pinkish orange) sapphires often draw higher prices than many of even the finest blue sapphires. Recently, more sapphires of this color have appeared on the market as a result of a new artificial treatment method that is called "lattice diffusion".^[7]

Padparadscha

Faceted padparadscha

Padparadscha is a delicate light to medium toned pink-orange to orange-pink hue corundum, originally found in Sri Lanka, but also found in deposits in Vietnam and parts of East Africa. Padparadscha sapphires are rare; the rarest of all is the totally natural variety, with no sign of artificial treatment.^[8]
The name is derived from the Sanskrit "padma raga" (padma = lotus; raga = color), a color akin to the lotus flower (Nelumbo nucifera ‘Speciosa’).^{[citation needed]}

Star sapphire

The Star of Bombay (182 carats (36 g))

A star sapphire is a type of sapphire that exhibits a star-like phenomenon known as asterism; red stones are known as "star rubies". Star sapphires contain intersecting needle-like inclusions following the underlying crystal structure that cause the appearance of a six-rayed "star"-shaped pattern when viewed with a single overhead light source. The inclusion is often the mineral rutile, a mineral composed primarily of titanium dioxide.^[9] The stones are cut en cabochon, typically with the center of the star near the top of the dome. Occasionally, twelve-rayed stars are found, or parallel whisker inclusions can produce a "cat's eye" effect.^[10]
The Black Star of Queensland, the largest gem-quality star sapphire in the world, weighs 733 carats.^[11] The Star of India (weighing 563.4 carats) is thought to be the second-largest star sapphire (the largest blue), and is currently on display at the American Museum of Natural History in New York City. The 182-carat Star of Bombay, located in the National Museum of Natural History, in Washington, D.C., is another example of a large blue star sapphire. The value of a star sapphire depends not only on the weight of the stone, but also the body color, visibility, and intensity of the asterism.

Color change sapphire

A rare variety of natural sapphire, known as color-change sapphire, exhibits different colors in different light. Color change sapphires are blue in outdoor light and purple under incandescent indoor light. Some stones shift color well and others only partially, in that some stones go from blue to bluish purple. While color change sapphires come from a variety of locations, the gem gravels of Tanzania is the main source.
Certain synthetic color-change "sapphires" are sold as “lab” or “synthetic” alexandrite, which more accurately is called an alexandrite simulant (also called alexandrium) since the latter is a type of chrysoberyl—an entirely different substance whose pleochroism is different and much more pronounced than color-change corundum (sapphire). These stones are not sapphires.

Source of color

Crystal structure of sapphire

Sapphire ring made circa 1940

Rubies are corundum which contain chromium impurities that absorb yellow-green light and result in deeper ruby red color with increasing content.^[12] Purple sapphires contain trace amounts of vanadium and come in a variety of shades. Corundum that contains ~0.01% of titanium is colorless. If trace amounts of iron are present, a very pale yellow to green color may be seen. If both titanium and iron impurities are present together, however, the result is a magnificent deep-blue color.^[13]
Unlike localized ("intra-atomic") absorption of light which causes color for chromium and vanadium impurities, blue color in sapphires comes from intervalence charge transfer, which is the transfer of an electron from one transition-metal ion to another via the conduction or valence band. The iron can take the form Fe²⁺ or Fe³⁺, while titanium generally takes the form Ti⁴⁺. If Fe²⁺ and Ti⁴⁺ ions are substituted for Al³⁺, localized areas of charge imbalance are created. An electron transfer from Fe²⁺ and Ti⁴⁺ can cause a change in the valence state of both. Because of the valence change there is a specific change in energy for the electron, and electromagnetic energy is absorbed. The wavelength of the energy absorbed corresponds to yellow light. When this light is subtracted from incident white light, the complementary color blue results. Sometimes when atomic spacing is different in different directions there is resulting blue-green dichroism.
Intervalence charge transfer is a process that produces a strong colored appearance at a low percentage of impurity. While at least 1% chromium must be present in corundum before the deep red ruby color is seen, sapphire blue is apparent with the presence of only 0.01% of titanium and iron.

Treatments

Sapphires may be treated by several methods to enhance and improve their clarity and color.^[14] It is common practice to heat natural sapphires to improve or enhance color. This is done by heating the sapphires in furnaces to temperatures between 500 and 1800 °C for several hours, or by heating in a nitrogen-deficient atmosphere oven for seven days or more. Upon heating, the stone becomes more blue in color, but loses some of the rutile inclusions (silk). When high temperatures are used, the stone loses all silk (inclusions) and it becomes clear under magnification.^[15] The inclusions in natural stones are easily seen with a jeweler's loupe. Evidence of sapphire and other gemstones being subjected to heating goes back at least to Roman times.^[16] Un-heated natural stones are somewhat rare and will often be sold accompanied by a certificate from an independent gemological laboratory attesting to "no evidence of heat treatment".

Yogo sapphire

Yogo sapphires sometimes do not need heat treating because their cornflower blue coloring is uniform and deep, they are generally free of the characteristic inclusions, and they have high uniform clarity.^[17] When Intergem Limited began marketing the Yogo in the 1980s as the world's only guaranteed untreated sapphire, heat treatment was not commonly disclosed; by 1982 the heat treatment became a major issue.^[18] At that time, 95% of all the world's sapphires were being heated to enhance their natural color.^[19] Intergem's marketing of guaranteed untreated Yogos set them against many in the gem industry. This issue appeared as a front page story in the Wall Street Journal on August 29, 1984 in an article by Bill Richards, Carats and Schticks: Sapphire Marketer Upsets The Gem Industry.^[19]
Diffusion treatments are used to add impurities to the sapphire to enhance color. Typically beryllium is diffused into a sapphire under very high heat, just below the melting point of the sapphire. Initially (c. 2000) orange sapphires were created, although now the process has been advanced and many colors of sapphire are often treated with beryllium. The colored layer can be removed when stones chip or are repolished or refaceted, depending on the depth of the impurity layer. Treated padparadschas may be very difficult to detect, and many stones are certified by gemological labs (e.g., Gubelin, SSEF, AGTA).
According to United States Federal Trade Commission guidelines, disclosure is required of any mode of enhancement that has a significant effect on the gem's value.^[20]

Mining

Sapphire from Madagascar

Sapphires are mined from alluvial deposits or from primary underground workings. Commercial mining locations for sapphire and ruby include (but are not limited to) the following countries: Afghanistan, Australia, Myanmar/Burma, Cambodia, China, Colombia, India, Kenya, Laos, Madagascar, Malawi, Nepal, Nigeria, Pakistan, Sri Lanka, Tajikistan, Tanzania, Thailand, USA, and Vietnam.^[21] The Logan sapphire, the Star of India, and the Star of Bombay originate from Sri Lankan mines. Madagascar is the world leader in sapphire production (as of 2007) specifically its deposits in and around the town of Ilakaka.^[22] Prior to the opening of the Ilakaka mines, Australia was the largest producer of sapphires (such as in 1987).^[23] In 1991 a new source of sapphires was discovered in Andranondambo, southern Madagascar. That area has been exploited for its sapphires started in 1993, but it was practically abandoned just a few years later – because of the difficulties in recovering sapphires in their bedrock.^[24]
In North America, sapphires have been mined mostly from deposits in Montana: fancies along the Missouri River near Helena, Montana, Dry Cottonwood Creek near Missoula, Montana, and Rock Creek near Philipsburg, Montana. Fine blue Yogo sapphires are found at Yogo Gulch west of Lewistown, Montana.^[25] A few gem-grade sapphires and rubies have also been found in the area of Franklin, North Carolina.^{[citation needed]}
The sapphire deposits of Kashmir are still well known in the gem industry,^[26] despite the fact that the peak production from this area mostly took place in a relatively short period at the end of the nineteenth and early twentieth centuries.^[27] At present, the world record price-per-carat for a sapphire at auction is held by a sapphire from Kashmir, which sold for more than $145,000 per carat (more than $3.8 million dollars in total) in November 2011.^[28]

Synthetic sapphire

Synthetic star sapphire

Synthetic sapphire

In 1902 the French chemist Auguste Verneuil developed a process for producing synthetic sapphire crystals.^[29] In the Verneuil process, named for him, fine alumina powder is added to an oxyhydrogen flame, and this is directed downward against a mantle.^[30] The alumina in the flame is slowly deposited, creating a teardrop shaped "boule" of sapphire material. Chemical dopants can be added to create artificial versions of the ruby, and all the other natural colors of sapphire, and in addition, other colors never seen in geological samples. Artificial sapphire material is identical to natural sapphire, except it can be made without the flaws that are found in natural stones. The disadvantage of Verneuil process is that the grown crystals have high internal strains. Many methods of manufacturing sapphire today are variations of the Czochralski process, which was invented in 1916.^[31] In this process a tiny sapphire seed crystal is dipped into a crucible made of the precious metal iridium or molybdenum,^[32] containing molten alumina, and then slowly withdrawn upward at a rate of one to 100 mm per hour. The alumina crystallizes on the end, creating long carrot-shaped boules of large size, up to 400 mm in diameter and weighing almost 500 kg.^[33]
Synthetic sapphire is industrially produced from agglomerated aluminium oxide, sintered and fused in an inert atmosphere (hot isostatic pressing for example), yielding a transparent polycrystalline product, slightly porous, or with more traditional methods such as Verneuil, Czochralski, flux method, etc., yielding a single crystal sapphire material which is non-porous and should be relieved of its internal stress.
In 2003 the world's production of synthetic sapphire was 250 tons (1.25 × 10⁹ carats), mostly by the United States and Russia.^[33][34] The availability of cheap synthetic sapphire unlocked many industrial uses for this unique material:
The first laser was made with a rod of synthetic ruby. Titanium-sapphire lasers are popular due to their relatively rare capacity to be tuned to various wavelengths in the red and near-infrared region of the electromagnetic spectrum. They can also be easily mode-locked. In these lasers a synthetically produced sapphire crystal with chromium or titanium impurities is irradiated with intense light from a special lamp, or another laser, to create stimulated emission.

Transparent and tough

One application of synthetic sapphire is sapphire glass. Here glass is a layman term which refers not to the amorphous state, but to the transparency. Sapphire is not only highly transparent to wavelengths of light between 150 nm (UV) and 5500 nm (IR) (the human eye can discern wavelengths from about 380 nm to 750 nm^[35]), but is also extraordinarily scratch-resistant (compare sapphire's scratch hardness of 400 versus 50 for window glass and 100 for quartz.^[36])

Common applications

Along with zirconia and aluminium oxynitride, synthetic sapphire is used for shatter resistant windows in armored vehicles and various military body armor suits, in association with composites.
A common use of synthetic sapphire is in sapphire optical windows. The key benefits of sapphire windows are:

• Very wide optical transmission band from UV to near-infrared, (0.15-5.5 µm)
• Significantly stronger than other optical materials or standard glass windows
• The hardest natural substance next to diamond
• Highly resistant to scratching and abrasion (9 Mohs scale)
• Extremely high melt temperature (2030°C)
• Totally unaffected by all chemicals except some very hot caustics

Sapphire glass windows (although being crystalline) are made from pure sapphire boules that have been grown in an application specific crystal orientation, typically along the optical axis, the c-axis, for standard optical windows for minimum birefringence. The boules are sliced up into the desired window thickness and finally polished to the desired surface finish. Sapphire optical windows can be polished to a wide range surface finishes due to its crystal structure and it hardness. The surface finishes of Optical Windows are normally called out by the Scratch-Dig specifications in accordance with the globally adopted MIL-O-13830 specification.
Sapphire glass windows are used in high pressure chambers for spectroscopy, crystals in various watches, and windows in grocery store barcode scanners since the material's exceptional hardness and toughness makes it very resistant to scratching.^[33]

Cermax xenon arc lamp with synthetic sapphire output window

One type of xenon arc lamp (originally called the "Cermax" its first brand name), which is now known generically as the "ceramic body xenon lamp", uses sapphire crystal output windows that tolerate higher thermal loads – and thus higher output powers when compared with conventional Xe lamps with pure silica window.^[37]

Use as substrate for semiconducting circuits

Thin sapphire wafers also are used as an insulating substrate in high-power, high-frequency CMOS integrated circuits. This type of IC is called a silicon on sapphire or "SOS" chip. These are especially useful for high-power radio-frequency (RF) applications such as those found in cellular telephones, police car and fire truck radios, and satellite communication systems. "SOS" allows for the monolithic integration of both digital and analog circuitry all on one IC chip.
The reason for choosing wafers of artificial sapphire, rather than some other substance, for these substrates is that sapphire has a quite low conductivity for electricity, but a much-higher conductivity for heat. Thus, sapphire provides good electrical insulation, while at the same time doing a good job at helping to conduct away the significant heat that is generated in all operating integrated circuits.
Once the single crystal sapphire boules are grown they are cored-sliced into cylindrical pieces. Wafers are then sliced from these cylindrical cores. These wafers of single-crystal sapphire material are also used in the semiconductor industry as a non-conducting substrate for the growth of devices based on gallium nitride (GaN). The use of the sapphire material significantly reduces the cost, because this has about one-seventh the cost of germanium. Gallium nitride on sapphire is commonly used in blue light-emitting diodes (LEDs).^[38]

Historical and cultural references

• Etymologically, the English word “sapphire” derives from Latin sapphirus, sappirus from Greek σαπφειρος (sappheiros) from Hebrew סַפִּיר (sappir) from Old Iranian sani-prijam, from Sanskrit, Shanipriya (शनिप्रिय), from "shani" (शनि) meaning "Saturn" and "priya" (प्रिय), dear, i.e. literally “dear to Saturn”.^[1]
• The Greek term for sapphire quite likely was misapplied, and instead, used to refer to lapis lazuli.^[1]
• During the Medieval Ages, European lapidaries came to refer to blue corundum crystal by its “sapphire-blue” color, whence the modern name for “sapphire”.^{[citation needed]}

Tourmaline

From Wikipedia, the free encyclopedia

Jump to: navigation, search

Tourmaline
Schorl Tourmaline
General
Category	Cyclosilicate
Formula (repeating unit)	(Ca,K,Na,[])(Al,Fe,Li,Mg,Mn)3(Al,Cr, Fe,V)6 (BO3)3(Si,Al,B)6O18(OH,F)4 [1][2]
Identification
Color	Most commonly black, but can range from brown, violet, green, pink, or in a dual-colored pink and green.
Crystal habit	Parallel and elongated. Acicular prisms, sometimes radiating. Massive. Scattered grains (in granite).
Crystal system	Trigonal
Cleavage	Indistinct
Fracture	Uneven, small conchoidal, brittle
Mohs scale hardness	7–7.5
Luster	Vitreous, sometimes resinous
Streak	White
Specific gravity	3.06 (+.20 -.06)[1]
Density	2.82–3.32
Polish luster	Vitreous[1]
Optical properties	Double refractive, uniaxial negative[1]
Refractive index	nω=1.635–1.675, nε=1.610–1.650
Birefringence	-0.018 to -0.040; typically about .020 but in dark stones it may reach .040[1]
Pleochroism	typically moderate to strong[1] Red Tourmaline: Definite; dark red,light red Green Tourmaline: Strong; dark green, yellow-green Brown Tourmaline: Definite; dark brown, light brown Blue Tourmaline: Strong; dark blue, light blue
Dispersion	.017[1]
Ultraviolet fluorescence	pink stones—inert to very weak red to violet in long and short wave[1]
Absorption spectra	a strong narrow band at 498 nm, and almost complete absorption of red down to 640nm in blue and green stones; red and pink stones show lines at 458 and 451nm as well as a broad band in the green spectrum[1]

Tourmaline (tur-mah-Leen) is a crystal boron silicate mineral compounded with elements such as aluminium, iron, magnesium, sodium, lithium, or potassium. Tourmaline is classified as a semi-precious stone and the gemstone comes in a wide variety of colors. The name comes from the Sinhalese word "Thuramali" (තුරමලි) or "Thoramalli" (තෝරමල්ලි), which applied to different gemstones found in Sri Lanka.

Contents 1 History 2 Tourmaline species and varieties 2.1 Schorl 2.2 Dravite 2.3 Elbaite 2.4 Chemical composition of the tourmaline group 3 Physical properties 3.1 Crystal structure 3.2 Color 3.3 Treatments 4 Geology 5 Tourmaline localities 5.1 United States 5.2 Brazil 5.2.1 World's largest 5.3 Africa 5.4 Afghanistan 6 Notes 7 References 8 Further reading 9 External links

History

Brightly colored Sri Lankan gem tourmalines were brought to Europe in great quantities by the Dutch East India Company to satisfy a demand for curiosities and gems. At the time it was not realised that schorl and tourmaline were the same mineral.

Tourmaline species and varieties

Commonly encountered species and varieties

• Schorl species:
  • Bluish or brownish black to Black—schorl
• Dravite species: from the Drave district of Carinthia
  • Dark yellow to brownish black—dravite
• Elbaite species: named after the island of Elba, Italy
  • Red or pinkish-red—rubellite variety (from ruby)
  • Light blue to bluish green—Brazilian indicolite variety (from indigo)
  • Green—verdelite or Brazilian emerald variety
  • Colorless—achroite variety (from the Greek "άχρωμος" meaning "colorless")

Schorl

The most common species of tourmaline is schorl. It may account for 95% or more of all tourmaline in nature. The early history of the mineral schorl shows that the name "schorl" was in use prior to 1400 because a village known today as Zschorlau (in Saxony, Germany) was then named "Schorl" (or minor variants of this name). This village had a nearby tin mine where, in addition to cassiterite, black tourmaline was found. The first description of schorl with the name "schürl" and its occurrence (various tin mines in the Saxony Ore Mountains) was written by Johannes Mathesius (1504–1565) in 1562 under the title "Sarepta oder Bergpostill".^[3] Up to about 1600, additional names used in the German language were "Schurel", "Schörle", and "Schurl". Beginning in the 18th century, the name Schörl was mainly used in the German-speaking area. In English, the names shorl and shirl were used in the 18th century. In the 19th century the names common schorl, schörl, schorl and iron tourmaline were the English words used for this mineral.^[3] The word tourmaline has two etymologies, both from the Sinhalese word turamali, meaning "stone attracting ash" (a reference to its pyroelectric properties) or according to other sources "mixed gemstones".

Dravite

Black Dravite on a grey matrix

The name dravite was used for the first time by Gustav Tschermak (1836–1927), Professor of Mineralogy and Petrography at the University of Vienna, in his book Lehrbuch der Mineralogie (published in 1884) for magnesium-rich (and sodium-rich) tourmaline from the village Unterdrauburg, Drava river area, Carinthia, Austro-Hungarian Empire. Today this tourmaline locality (type locality for dravite) at the village Dravograd (near Dobrova pri Dravogradu), is a part of the Republic of Slovenia.^[4] Tschermak gave this tourmaline the name dravite, for the Drava river area, which is the district along the Drava River (in German: Drau, in Latin: Drave) in Austria and Slovenia. The chemical composition which was given by Tschermak in 1884 for this dravite approximately corresponds to the formula NaMg₃(Al,Mg)₆B₃Si₆O₂₇(OH), which is in good agreement (except for the OH content) with the endmember formula of dravite as known today.^[4]

Elbaite

A lithium-tourmaline (elbaite) was one of three pegmatitic minerals from Utö, Sweden, in which the new alkali element lithium (Li) was determined in 1818 by Johan August Arfwedson for the first time.^[5] Elba Island, Italy, was one of the first localities where colored and colorless Li-tourmalines were extensively chemically analysed. In 1850 Karl Friedrich August Rammelsberg described fluorine (F) in tourmaline for the first time. In 1870 he proved that all varieties of tourmaline contain chemically bound water. In 1889 Scharitzer proposed the substitution of (OH) by F in red Li-tourmaline from Sušice, Czech Republic. In 1914 Vladimir Vernadsky proposed the name Elbait for lithium-, sodium-, and aluminum-rich tourmaline from Elba Island, Italy, with the simplified formula (Li,Na)HAl₆B₂Si₄O₂₁.^[5] Most likely the type material for elbaite was found at Fonte del Prete, San Piero in Campo, Campo nell'Elba, Elba Island, Province of Livorno, Tuscany, Italy.^[5] In 1933 Winchell published an updated formula for elbaite, H₈Na₂Li₃Al₃B₆Al₁₂Si₁₂O₆₂, which is commonly used to date written as Na(Li_1.5Al_1.5)Al₆(BO₃)₃[Si₆O₁₈](OH)₃(OH).^[5] The first crystal structure determination of a Li-rich tourmaline was published in 1972 by Donnay and Barton, performed on a pink elbaite from San Diego County, California, United States.

Chemical composition of the tourmaline group

The tourmaline mineral group is chemically one of the most complicated groups of silicate minerals. Its composition varies widely because of isomorphous replacement (solid solution), and its general formula can be written as
XY₃Z₆(T₆O₁₈)(BO₃)₃V₃W,
where:^[6]
X = Ca, Na, K, ៛ = vacancy
Y = Li, Mg, Fe²⁺, Mn²⁺, Zn, Al, Cr³⁺, V³⁺, Fe³⁺, Ti⁴⁺, vacancy
Z = Mg, Al, Fe³⁺, Cr³⁺, V³⁺
T = Si, Al, B
B = B, vacancy
V = OH, O
W = OH, F, O

Large pink elbaite crystal on quartz, Cryo-Genie Mine, San Diego Co., California, US.

The 25 minerals in the group (endmember formulas) recognized by the International Mineralogical Association

Chromium-dravite	NaMg3Cr6Si6O18(BO3)3(OH)3OH
Chromo-alumino-povondraite	NaCr3(Al4Mg2)Si6O18(BO3)3(OH)3O
Darrellhenryite	NaLiAl2Al6Si6O18(BO3)3(OH)3O
Dravite	NaMg3Al6Si6O18(BO3)3(OH)3OH
Elbaite	Na(Li1.5,Al1.5)Al6Si6O18(BO3)3(OH)3OH
Feruvite	CaFe2+3(MgAl5)Si6O18(BO3)3(OH)3OH
Fluor-buergerite	NaFe3+3Al6Si6O18(BO3)3O3F
Fluor-dravite	NaMg3Al6Si6O18(BO3)3(OH)3F
Fluor-elbaite	Na(Li1.5,Al1.5)Al6Si6O18(BO3)3(OH)3F
Fluor-liddicoatite	Ca(Li2Al)Al6Si6O18(BO3)3(OH)3F
Fluor-schorl	NaFe2+3Al6Si6O18(BO3)3(OH)3F
Fluor-tsilaisite	NaMn2+3Al6Si6O18(BO3)3(OH)3F
Foitite	៛(Fe2+2Al)Al6Si6O18(BO3)3(OH)3OH
Magnesiofoitite	៛(Mg2Al)Al6Si6O18(BO3)3(OH)3OH
Olenite	NaAl3Al6Si6O18(BO3)3O3OH
Oxy-chromium-dravite	NaCr3(Mg2Cr4)Si6O18(BO3)3(OH)3O
Oxy-dravite	Na(Al2Mg)(Al5Mg)Si6O18(BO3)3(OH)3O
Oxy-schorl	Na(Fe2+2Al)Al6Si6O18(BO3)3(OH)3O
Oxy-vanadium-dravite	NaV3(Al4Mg2)Si6O18(BO3)3(OH)3O
Povondraite	NaFe3+3(Fe3+4Mg2)Si6O18(BO3)3(OH)3O
Rossmanite	៛(LiAl2)Al6Si6O18(BO3)3(OH)3OH
Schorl	NaFe2+3Al6Si6O18(BO3)3(OH)3OH
Tsilaisite	NaMn2+3Al6Si6O18(BO3)3(OH)3OH
Uvite	CaMg3(Al5Mg)Si6O18(BO3)3(OH)3OH
Vanadio-oxy-chromium-dravite	NaV3(Cr4Mg2)Si6O18(BO3)3(OH)3O

A revised nomenclature for the tourmaline group was published in 2011.^[7][8][9]

Tri-color elbaite crystals on quartz, Himalaya Mine, San Diego Co., California, US

Physical properties

Crystal structure

Tourmaline belongs to the trigonal crystal system and occurs as long, slender to thick prismatic and columnar crystals that are usually triangular in cross-section. The style of termination at the ends of crystals is asymmetrical, called hemimorphism. Small slender prismatic crystals are common in a fine-grained granite called aplite, often forming radial daisy-like patterns. Tourmaline is distinguished by its three-sided prisms; no other common mineral has three sides. Prisms faces often have heavy vertical striations that produce a rounded triangular effect. Tourmaline is rarely perfectly euhedral. An exception was the fine dravite tourmalines of Yinnietharra, in western Australia. The deposit was discovered in the 1970s, but is now exhausted. All hemimorphic crystals are piezoelectric, and are often pyroelectric as well.

Color

Tourmaline gemstones - Mozambique

Tourmaline has a variety of colors. Usually, iron-rich tourmalines are black to bluish-black to deep brown, while magnesium-rich varieties are brown to yellow, and lithium-rich tourmalines are almost any color: blue, green, red, yellow, pink, etc. Rarely, it is colorless. Bi-colored and multicolored crystals are common, reflecting variations of fluid chemistry during crystallization. Crystals may be green at one end and pink at the other, or green on the outside and pink inside; this type is called watermelon tourmaline. Some forms of tourmaline are dichroic, in that they change color when viewed from different directions.
The pink color of tourmalines from many fields is the result of prolonged natural irradiation. During their growth, these tourmaline crystals incorporated Mn²⁺ and were initially very pale. Due to natural gamma ray exposure from radioactive decay of ⁴⁰K in their granitic environment, gradual formation of Mn³⁺ ions occurs, which is responsible for the deepening of the pink to red color.^[10]

Treatments

Some tourmaline gems, especially pink to red colored stones, are altered by irradiation to improve their color. Irradiation is almost impossible to detect in tourmalines, and does not, currently, impact the value. Heat treatment is also used to enhance tourmaline. Heavily-included tourmalines, such as rubellite and Brazilian paraiba, are sometimes clarity-enhanced. A clarity-enhanced tourmaline (especially paraiba) is worth much less than a non-treated gem.^[11]

Geology

Tourmaline is found in granite and granite pegmatites and in metamorphic rocks such as schist and marble. Schorl and lithium-rich tourmalines are usually found in granite and granite pegmatite. Magnesium-rich tourmalines, dravites, are generally restricted to schists and marble. Tourmaline is a durable mineral and can be found in minor amounts as grains in sandstone and conglomerate, and is part of the ZTR index for highly-weathered sediments.

Bi-colored tourmaline crystal, 0.8 inches (2 cm) long.

Tourmaline localities

Gem and specimen tourmaline is mined chiefly in Brazil and Africa. Some placer material suitable for gem use comes from Sri Lanka. In addition to Brazil, tourmaline is mined in Tanzania, Nigeria, Kenya, Madagascar, Mozambique, Namibia, Afghanistan, Pakistan, Sri Lanka, and Malawi.^[12]

United States

Some fine gems and specimen material has been produced in the United States, with the first discoveries in 1822, in the state of Maine. California became a large producer of tourmaline in the early 1900s. The Maine deposits tend to produce crystals in raspberry pink-red as well as minty greens. The California deposits are known for bright pinks, as well as bicolors. During the early 1900s, Maine and California were the world's largest producers of gem tourmalines. The Empress Dowager Tz'u Hsi of China loved pink tourmaline and bought large quantities for gemstones and carvings from the then new Himalaya Mine, located in San Diego County, California.^[13] It is not clear when the first tourmaline was found in California. Native Americans have used pink and green tourmaline as funeral gifts for centuries. The first documented case was in 1890 when Charles Russel Orcutt found pink tourmaline at what later became the Stewart Mine at Pala, San Diego.^[14]

Brazil

Watermelon Tourmaline mineral on quartz matrix (crystal approximately 2 cm wide at face)

Almost every color of tourmaline can be found in Brazil, especially in the Brazilian states of Minas Gerais and Bahia. In 1989, miners discovered a unique and brightly colored variety of tourmaline in the state of Paraíba.^{[citation needed]} The new type of tourmaline, which soon became known as paraiba tourmaline, came in blue and green. Brazilian paraiba tourmaline is usually contains abundant inclusions. Much of the paraiba tourmaline from Brazil actually comes from the neighboring state of Rio Grande do Norte. Material from Rio Grande do Norte is often somewhat less intense in color, but many fine gems are found there. It was determined that the element copper was important in the coloration of the stone.^[15]

World's largest

A large cut tourmaline from Paraiba, measuring 36.44 x 33.75 x 21.85 mm (1.43 x 1.33 x 0.86 in) and weighing 191.87 carats, was included in the Guinness World Records.^[16] The large natural gem, owned by Billionaire Business Enterprises,^[16] is a bluish-green in color. The flawless oval shaped cut stone was presented in Montreal, Quebec, Canada on 14 October 2009.^[17]

Africa

This section does not cite any references or sources. Please help improve this section by adding citations to reliable sources. Unsourced material may be challenged and removed. (September 2012)

In the late 1990s, copper-containing tourmaline was found in Nigeria. The material was generally paler and less saturated than the Brazilian materials, although the material generally was much less included. A more recent African discovery from Mozambique has also produced beautiful tourmaline colored by copper, similar to the Brazilian paraiba. While its colors are somewhat less bright than top Brazilian material, Mozambique paraiba is often less included and has been found in larger sizes. The Mozambique paraiba material usually is more intensely colored than the Nigerian. There is a significant overlap in color and clarity with Mozambique paraiba and Brazilian paraiba, especially with the material from Rio Grande do Norte. While less expensive than top quality Brazilian paraiba, some Mozambique material sells for well over $5,000 per carat, which still is extremely high compared to other tourmalines.

Tourmaline mineral (approximately 10 cm tall)

Another highly valuable variety is chrome tourmaline, a rare type of dravite tourmaline from Tanzania. Chrome tourmaline is a rich green color due to the presence of chromium atoms in the crystal; chromium also produces the green color of emeralds. Of the standard elbaite colors, blue indicolite gems are typically the most valuable, followed by green verdelite and pink to red rubellite.^{[citation needed]} There are also yellow tourmalines, sometimes known as canary tourmaline. Zambia is rich in both red and yellow tourmaline, which are relatively inexpensive in that country. Ironically the rarest variety, colorless achroite, is not appreciated and is the least expensive of the transparent tourmalines.

Afghanistan

Extra fine indicolite (blue tourmaline) and verdelite (green tourmaline) are found in the Nuristan region (Ghazi Abad district) and Pech Valley (Pech and Chapa Dara districts) of Kunar province. Gem-quality tourmalines are faceted (cut) from 0.50–10 gram sizes and have unusually high clarity and intense shades of color.

Notes

1. ^ ^{a b c d e f g h i} Gemological Institute of America, GIA Gem Reference Guide 1995, ISBN 0-87311-019-6
2. ^ Mindat tourmaline group Accessed September 12, 2005. This website details specifically and clearly how the complicated chemical formula is structured.
3. ^ ^{a b} Ertl, 2006.
4. ^ ^{a b} Ertl, 2007.
5. ^ ^{a b c d} Ertl, 2008.
6. ^ Hawthorne, F.C. & Henry, D.J. (1999). "Classification of the minerals of the tourmaline group". European Journal of Mineralogy, 11, pp. 201–215.
7. ^ Darrell J. Henry, Milan Novák, Frank C. Hawthorne, Andreas Ertl, Barbara L. Dutrow, Pavel Uher, and Federico Pezzotta (2011). "Nomenclature of the tourmaline-supergroup minerals". American Mineralogist 96: 895–913. doi:10.2138/am.2011.3636.
8. ^ Erratum: American Mineralogist (2013), Volume 98, page 524.
9. ^ Frank C. Hawthorne and Dona M. Dirlam. "Tourmaline: Tourmaline the Indicator Mineral: From Atomic Arrangement to Viking Navigation." Elements, October 2011, v. 7, p. (5): 307-312, doi:10.2113/gselements.7.5.307.
10. ^ Reinitz & Rossman, 1998.
11. ^ Kurt Nassau (1984), Gemstone Enhancement: Heat, Irradiation, Impregnation, Dyeing, and Other Treatments, Butterworth Publishers
12. ^ Hurlbut and Klien. Manual of Mineralogy (after Dana), 19th Edition, John Wiley and Sons, Publishers
13. ^ Fred Rynerson, Exploring and Mining Gems and Gold in the West, Naturegraph Publishers.
14. ^ Paul Willard Johnson, "Common Gems of San Diego," Gems and Gemology, Vol. XII, Winter 1968-69, p. 358.
15. ^ Rossman et al. 1991.
16. ^ ^{a b} Guinness World Records Official Website
17. ^ Giant jewel breaks record

References

• Ertl, A., Pertlik, F. & Bernhardt, H.-J. (1997) Investigations on olenite with excess boron from the Koralpe, Styria, Austria, Österreichische Akademie der Wissenschaften, Mathematisch-Naturwissenschaftliche Klasse, Abt. I, Anzeiger, 134, pp 3–10. Article Online
• Ertl, A. (2006) About the etymology and the type localities of schorl Mitteilungen der Österreichischen Mineralogischen Gesellschaft, 152, 2006, pp 7–16. Article Online
• Ertl, A. (2007) About the type locality and the nomenclature of dravite Mitteilungen der Österreichischen Mineralogischen Gesellschaft, 153, 2007, pp 265–271. Article Online
• Ertl, A. (2008) About the nomenclature and the type locality of elbaite: A historical review Mitteilungen der Österreichischen Mineralogischen Gesellschaft, 154, 2008, pp 35–44. Article Online
• Reinitz, I.M. & Rossman G.R. (1988) Role of natural radiation in tourmaline coloration. American Mineralogist, 73, pp 822-825. Article Online
• Rossman, G.R., Fritsch E., & Shigley J.E. (1991) Origin of color in cuprian elbaite from São José de Batalha, Paraíba, Brazil. American Mineralogist, 76, pp 1479-1484. Article Online
• Schumann, Walter (2006). Gemstones of the World 3rd Edition. Sterling Publishing, New York; pp 126–127.

Further reading

• Darrell J. Henry, Milan Novák, Frank C. Hawthorne, Andreas Ertl, Barbara L. Dutrow, Pavel Uher, and Federico Pezzotta (2011). "Nomenclature of the tourmaline-supergroup minerals". American Mineralogist 96: 895–913. doi:10.2138/am.2011.3636.

External links

Wikimedia Commons has media related to: Tourmaline

Look up tourmaline in Wiktionary, the free dictionary.

• Tourmaline classification Accessed October 8, 2011
• Mindat tourmaline group Accessed October 8, 2011
• ICA's tourmaline page International Colored Stone Association on tourmaline
• Farlang historical tourmaline references US localities, antique references,
• Webmineral elbaite page, crystallographic and mineral information on elbaite
• Tourmaline Characteristics and Geological data

[hide] v t e Jewellery
Forms	Anklet Belt buckle Belly chain Bindi Bracelet Brooch Chatelaine Collar pin Crown Cufflink Earring Lapel pin Necklace Pendant Ring Tiara Tie clip Tie pin Toe ring Watch pocket
Making	People Bench jeweler Clockmaker Goldsmith Silversmith Jewelry designer Lapidary Watchmaker Processes Casting centrifugal lost-wax vacuum Enameling Engraving Filigree Metal clay Plating Polishing Repoussé and chasing Soldering Stonesetting Wire sculpture Wire wrapped jewelry Tools Draw plate File Hammer Mandrel Pliers
Materials	Precious metals Gold Palladium Platinum Rhodium Silver Precious metal alloys Britannia silver Colored gold Crown gold Electrum Platinum sterling Shakudō Shibuichi Sterling silver Tumbaga Base metals Brass Bronze Copper Mokume-gane Pewter Stainless steel Titanium Tungsten Mineral gemstones Aventurine Agate Amethyst Beryl Carnelian Chrysoberyl Diamond Diopside Emerald Garnet Jade Jasper Lapis lazuli Larimar Malachite Marcasite Moonstone Obsidian Onyx Opal Peridot Quartz Ruby Sapphire Sodalite Sunstone Tanzanite Tiger's eye Topaz Tourmaline Turquoise Yogo sapphire Organic gemstones Abalone Amber Ammolite Copal Coral Ivory Jet Pearl Nacre Other natural objects Shell jewelry
Terms	Carat (mass) Carat (purity) Finding Millesimal fineness