The 423-carat (85 g) blue "Logan Sapphire
|"Aluminium oxide, "Al2"O3|
|"Crystal class||Hexagonal scalenohedral (3m)
"H-M symbol: (32/m)
|Color||Typically blue, but varies|
|"Crystal habit||As crystals, massive and granular|
|"Mohs scale hardness||9.0|
|Optical properties||"Abbe number 72.2|
|"Melting point||2,030–2,050 °C|
"Coefficient of thermal expansion (5.0–6.6)×10−6/K
ε = 8.9–11.1 (anisotropic)
Sapphire is a precious "gemstone, a variety of the mineral "corundum, an "aluminium oxide (α-"Al2"O3). It is typically blue, but natural "fancy" sapphires also occur in yellow, purple, orange, and green colors; "parti sapphires" show two or more colors. The only color which sapphire cannot be is red – as red colored corundum is called "ruby, another corundum variety. Pink colored corundum may be either classified as ruby or sapphire depending on locale. This variety in color is due to trace amounts of elements such as "iron, "titanium, "chromium, "copper, or "magnesium.
Commonly, natural sapphires are cut and polished into gemstones and worn in "jewelry. They also may be created synthetically in laboratories for industrial or decorative purposes in large "crystal boules. Because of the remarkable "hardness of sapphires – 9 on the "Mohs scale (the third hardest mineral, after "diamond at 10 and "moissanite at 9.5) – sapphires are also used in some non-ornamental applications, such as "infrared "optical components, 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 (especially integrated circuits and GaN-based LEDs).
Sapphire is one of the two gem-varieties of "corundum, the other being "ruby (defined as corundum in a shade of red). Although blue is the best-known sapphire color, they occur in other colors, including gray and black, and they can be colorless. A pinkish orange variety of sapphire is called "padparadscha.
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.:164–166 Sapphire and rubies are often found in the same geological setting.
Every sapphire mine produces a wide range of quality – and origin is not a guarantee of quality. For sapphire, "Kashmir receives the highest premium although Burma, Sri Lanka, and Madagascar also produce large quantities of fine quality gems.
The cost of natural sapphires varies depending on their color, clarity, size, cut, and overall quality. For gems of exceptional quality, an independent determination from a respected laboratory such as the "GIA, "AGL or Gubelin of origin often adds to value.
Gemstone color can be described in terms of "hue, "saturation, and "tone. Hue is 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.:18–22 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.:163–169 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. 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.:163–169
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.:18–22
Sapphires in colors other than blue are called "fancy" or "parti colored" sapphires.
Fancy sapphires are often found in yellow, orange, green, brown, purple and violet hues.
Particolored sapphires are those stones which exhibit two or more colors within a single stone. Australia is the largest source of particolored sapphires; they are not commonly used in mainstream jewelry and remain relatively unknown. Particolored sapphires cannot be created synthetically and only occur naturally.["citation needed]
Colorless sapphires have historically been used as diamond substitutes in jewelry.
Pink sapphires occur in shades from light to dark pink, and deepen in color as the quantity of "chromium increases. The deeper the pink color, the higher their monetary value. In the United States, a minimum color saturation must be met to be called a ruby, otherwise the stone is referred to as a pink sapphire.
Padparadscha is a delicate, light to medium toned, pink-orange to orange-pink hued "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.
Natural padparadscha 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 called "lattice diffusion".
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 causes 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. 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, typically because two different sets of inclusions are found within the same stone, such as a combination of fine needles of rutile with small platelets of "hematite; the first results in a whitish star and the second results in a golden-colored star. During crystallisation, the two types of inclusions become preferentially oriented in different directions within the crystal, thereby forming two six-rayed stars that are superimposed upon each other to form a twelve-rayed star. Misshapen stars or 12-rayed stars may also form as a result of "twinning. The inclusions can alternatively produce a ""cat's eye" effect if the 'face-up' direction of the cabochon's dome is oriented perpendicular to the crystal's c-axis rather than parallel to it. If the dome is oriented in between these two directions, an 'off-center' star will be visible, offset away from the high point of the dome.
"The Star of Adam is the largest blue star sapphire which weighs 1404.49 carats. The gem was mined in the city of Ratnapura, southern Sri Lanka. The "Black Star of Queensland, the second largest gem-quality star sapphire in the world, weighs 733 "carats. The "Star of India mined in Sri Lanka and weighing 563.4 carats is thought to be the third-largest star sapphire, and is currently on display at the "American Museum of Natural History in "New York City. The 182-carat "Star of Bombay, mined in Sri Lanka and 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.["citation needed]
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, or green to gray-green in daylight and pink to reddish-violet in incandescent light. Color change sapphires come from a variety of locations, including "Thailand and "Tanzania. The color-change effect is caused by the interaction of the sapphire, which absorbs specific wavelengths of light, and the light-source, whose spectral output varies depending upon the illuminant. Transition-metal impurities in the sapphire, such as chromium and vanadium, are responsible for the color change.
Certain synthetic color-change sapphires have a similar color change to the natural gemstone "alexandrite and they are sometimes marketed as "alexandrium" or "synthetic alexandrite". However, the latter term is a misnomer: synthetic color-change sapphires are, technically, not synthetic alexandrites but rather alexandrite simulants. This is because genuine alexandrite is a variety of "chrysoberyl: not sapphire, but an entirely different mineral.
"Rubies are "corundum which contain "chromium impurities that absorb yellow-green light and result in deeper "ruby red color with increasing content. 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. However, if both titanium and iron impurities are present together, and in the correct "valence states, the result is a deep-blue color.
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.
Sapphires can be treated by several methods to enhance and improve their clarity and color.:169 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 1,800 °C (932 and 3,272 °F) 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. 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. 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 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. 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. At that time, 95% of all the world's sapphires were being heated to enhance their natural color. 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 29 August 1984 in an article by Bill Richards, Carats and Schticks: Sapphire Marketer Upsets The Gem Industry.
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).
There are several ways of treating sapphire. Heat-treatment in a reducing or oxidising atmosphere (but without the use of any other added impurities) is commonly used to improve the color of sapphires, and this process is sometimes known as "heating only" in the gem trade. In contrast, however, heat treatment combined with the deliberate addition of certain specific impurities (e.g. beryllium, titanium, iron, chromium or nickel, which are absorbed into the crystal structure of the sapphire) is also commonly performed, and this process can be known as "diffusion" in the gem trade. However, despite what the terms "heating only" and "diffusion" might suggest, both of these categories of treatment actually involve diffusion processes.
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, United States, and "Vietnam. Sapphires from different geographic locations may have different appearances or chemical-impurity concentrations, and tend to contain different types of microscopic inclusions. Because of this, sapphires can be divided into three broad categories: classic metamorphic, non-classic metamorphic or magmatic, and classic magmatic.
Sapphires from certain locations, or of certain categories, may be more commercially appealing than others, particularly classic metamorphic sapphires from "Kashmir, Burma, or Sri Lanka that have not been subjected to heat-treatment.
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. Prior to the opening of the Ilakaka mines, Australia was the largest producer of sapphires (such as in 1987). 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.
In "North America, sapphires have been mined mostly from deposits in "Montana: fancies along the "Missouri River near "Helena, Montana, Dry Cottonwood Creek near "Deer Lodge, Montana, and Rock Creek near "Philipsburg, Montana. Fine blue "Yogo sapphires are found at Yogo Gulch west of "Lewistown, Montana. A few gem-grade sapphires and rubies have also been found in the area of "Franklin, North Carolina.
The sapphire deposits of "Kashmir are well known in the gem industry, despite the fact their peak production took place in a relatively short period at the end of the nineteenth and early twentieth centuries. They have a superior cornflower blue hue to them with a mysterious and almost sleepy quality, described by some gem enthusiasts as ‘blue velvet”. Kashmir-origin contributes meaningfully to the value of a sapphire, and most corundum of Kashmir origin can be readily identified by its characteristic silky appearance and exceptional hue. The unique blue appears lustrous under any kind of light, unlike non-Kashmir sapphires which may appear purplish or grayish in comparison. Sotheby's has been in the forefront overseeing record-breaking sales of Kashmir sapphires worldwide. In October 2014, Sotheby’s Hong Kong achieved consecutive per-carat price records for Kashmir sapphires - first with the 12.00 carat Cartier sapphire ring at US$193,975 per carat, then with a 17.16 carat sapphire at US$236,404, and again in June 2015 when the per-carat auction record was set at US$240,205. At present, the world record price-per-carat for sapphire at auction is held by a sapphire from Kashmir in a ring, which sold in October 2015 for approximately "US$242,000 per carat ("HK$52,280,000 in total, including buyer's premium, or more than US$6.74 million).
In 1902, the French chemist "Auguste Verneuil developed a process for producing synthetic sapphire crystals. In the "Verneuil process, named after him, fine "alumina powder is added to an "oxyhydrogen flame, and this is directed downward against a mantle. 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 by Polish chemist "Jan Czochralski. In this process, a tiny sapphire seed crystal is dipped into a crucible made of the precious metal "iridium or "molybdenum, containing molten alumina, and then slowly withdrawn upward at a rate of 1 to 100 mm per hour. The alumina crystallizes on the end, creating long carrot-shaped boules of large size up to 200 kg in mass.
Synthetic sapphire is also produced industrially from agglomerated aluminium oxide, sintered and fused (such as by "hot isostatic pressing) in an inert atmosphere, yielding a transparent but slightly "porous "polycrystalline product.["citation needed]
In 2003, the world's production of synthetic sapphire was 250 tons (1.25 × 109 carats), mostly by the United States and Russia. 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.
Synthetic sapphire—sometimes referred to as sapphire glass—is commonly used as a window material, because it is both highly transparent to wavelengths of light between 150 nm ("UV) and 5500 nm ("IR) (the visible spectrum extends about 380 nm to 750 nm), and extraordinarily scratch-resistant.
The key benefits of sapphire windows are:
Some sapphire-glass windows are made from pure sapphire boules that have been grown in a specific crystal orientation, typically along the optical axis, the c-axis, for minimum "birefringence for the application.
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 of surface finishes due to its crystal structure and its 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.["clarification needed]
The sapphire windows are used in both high pressure and vacuum 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.
It is used for end windows on some high-powered laser tubes as its wide-band transparency and thermal conductivity allow it to handle very high power densities in the infra-red or UV spectrum without degrading due to heating.
One type of "xenon arc lamp – originally called the "Cermax" and now known generically as the "ceramic body xenon lamp" – uses sapphire crystal output windows. This product tolerates higher thermal loads and thus higher output powers when compared with conventional Xe lamps with pure silica window.
Thin sapphire wafers were the first successful use of an insulating "substrate upon which to deposit silicon to make the "integrated circuits known as "silicon on sapphire or "SOS"; now other substrates can also be used for the class of circuits known more generally as "silicon on insulator. Besides its excellent electrical insulating properties, sapphire has high "thermal conductivity. "CMOS chips on sapphire are especially useful for high-power radio-frequency (RF) applications such as those found in "cellular telephones, "public-safety band radios, and "satellite communication systems. "SOS" also allows for the monolithic integration of both "digital and "analog circuitry all on one IC chip, and the construction of extremely low power circuits.
In one process, after single crystal sapphire boules are grown, they are core-drilled into cylindrical rods, and wafers are then sliced from these cores.["citation needed]
Wafers of single-crystal sapphire are also used in the "semiconductor industry as "substrates for the growth of devices based on "gallium nitride (GaN). The use of sapphire significantly reduces the cost, because it has about one-seventh the cost of germanium. Gallium nitride on sapphire is commonly used in blue "light-emitting diodes (LEDs).
Monocrystalline sapphire is fairly biocompatible and the exceptionally low wear of sapphire–metal pairs has led to the introduction (in Ukraine) of sapphire monocrystals for hip joint endoprostheses.
|"Bismarck Sapphire||"Myanmar||98.56 carats||Table||Blue||"National Museum of Natural History, Washington|
|"Black Star of Queensland||Australia, 1938||733 carats||Star||Black||Anonymous owner|
|"Logan Sapphire||Sri Lanka||422.99 carats||Cushion||Blue||"National Museum of Natural History, Washington|
|"Queen Marie of Romania||Sri Lanka||478.68 carats||Cushion||Blue||Anonymous owner|
|"Star of Adam||Sri Lanka, 2015||1404.49 carats||Star||Blue||Anonymous owner|
|"Star of Bombay||Sri Lanka||182 carats||Star||Blue-violet||"National Museum of Natural History, Washington|
|"Star of India||Sri Lanka||563.4 carats||Star||Blue-gray||"American Museum of Natural History, New York|
|"Stuart Sapphire||Sri Lanka||104 carats||Blue||"Tower of London|
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