Transparent to translucent natural or synthetic variety of corundum that is highly prized as a gemstone. Its colour is due mainly to the presence of small amounts of iron and titanium and normally ranges from very pale blue to deep indigo. Colourless, gray, yellow, pale pink, orange, green, violet, and brown varieties also are known as sapphire; red varieties are called ruby. Synthetic sapphire has been produced commercially since 1902. Much is used in jewelry, but most is used in the manufacture of jewel bearings, gauges, dies, and other specialized components; some also is used as a high-grade abrasive. It is found in Sri Lanka, Myanmar, India, and Montana in the U.S.
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The cost of sapphire gems varies depending on their color, clarity, size, cut, and overall quality. As of 2000, the cost of 1 carat (0.2 g) of a typical uncut, gem quality sapphire was about $1,600 USD. Significant sapphire deposits are found in Eastern Australia, Southeast Asia, and Sri Lanka. Sapphire and rubies are often found together in the same area, but one gem is usually more abundant.
Various shades of blue [dark and light] result from titanium and iron substitutions in the aluminum oxide crystal lattice. Some stones are not well saturated and show tones of gray. It is common practice to bake natural sapphires to improve or enhance color. This is usually done by heating the sapphires to temperatures of up to 1800 °C for several hours, or by heating in a nitrogen-deficient atmosphere oven for seven days or more. On magnification, the silk due to included rutile needles are often visible. If the needles are unbroken, then the stone was not heated; if the silk is not visible then the stone was heated adequately. If the silk is partially broken, then a process known as low tube heat may have been used. Low tube heat is the process whereby the rough stone is heated to 1300 °C over charcoal for 20 to 30 minutes. This removes gray or brown in the stone and improves color saturation.
A star sapphire is a type of sapphire that exhibits a star-like phenomenon known as asterism. Star sapphires contain intersecting needle-like inclusions (often the mineral rutile, a mineral composed primarily of titanium dioxide) that cause the appearance of a six-rayed 'star'-shaped pattern when viewed with a single overhead light source.
The value of a star sapphire depends not only on the carat weight of the stone but also the body color, visibility and intensity of the asterism.
Sapphires are mined from alluvial deposits or from primary underground workings. The finest specimens are mined in the disputed territory of Kashmir, as well as Myanmar, Madagascar, and Sri Lanka. Both the Logan sapphire and the Star of Bombay originate from Sri Lankan mines. Sapphires are also mined in Australia, Thailand, and China. Madagascar leads the world in sapphire production (as of 2007) specifically in and around the city of Ilakaka. Prior to Ilakaka, Australia was the largest producer of sapphires (as of 1987). Ilakaka is prone to violence, but sapphires are found everywhere including on the ground and in the river mud. Pakistan, Afghanistan, India, Tanzania, and Kenya also produce sapphires, and less commercially-significant deposits are found in many other countries. The US state of Montana has produced sapphires from both the El Dorado Bar and Spokane Bar deposit near Helena. Well-known for their intense, pure blue color, Yogo sapphires are found in Yogo Gulch, near Utica, Montana. Gem grade sapphires and rubies are also found in and around Franklin, North Carolina, USA. Several mines are open to the public.
In 1902, French chemist Auguste Verneuil developed a process for growing synthetic sapphire crystals. In the Verneuil process, fine alumina powder is added to an oxyhydrogen flame which is directed against a mantle. Alumina in the flame is slowly deposited, creating a teardrop shaped 'boule' of sapphire. Chemical dopants can be added to create artificial versions of ruby and all the other sapphire gems, plus colors never seen in nature. Artificial sapphire is identical to natural sapphire, except it can be made without the flaws found in natural stones. Many methods of manufacturing sapphire today are variations of the Czochralski process, invented in 1916. A tiny sapphire seed crystal is dipped into a crucible of molten alumina and 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 400 mm in diameter and weighing almost 500 kg.
As of 2003, the world's production capacity of synthetic sapphire is 250 tons.(1.25 x 109carats). 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 the relatively rare ability to tune the laser wavelength in the red-to near infrared region of the electromagnetic spectrum. They can also be easily modelocked. 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.
One application of synthetic sapphire is sapphire glass. Sapphire is not only highly transparent to wavelengths of light between 170 nm to 5.3 μm (the human eye can discern wavelengths from around 400 nm to 700 nm), but it is also five times stronger than glass and ranks a 9 on the Mohs Scale, although it is also more brittle. Sapphire glass is made from pure sapphire boules by slicing off and polishing thin wafers. Sapphire glass windows are used in high pressure chambers for spectroscopy, crystals in high quality watches, and windows in grocery store barcode scanners since the material's exceptional hardness makes it very resistant to scratching. Owners of such watches should still be careful to avoid exposure to diamond jewelry, and should avoid striking their watches against artificial stone and simulated stone surfaces that often contain silicon carbide and other materials that are harder than sapphire and thus capable of causing scratches.
One type of xenon arc lamp, known as Cermax (original brand name — generically known as a ceramic body xenon lamp), uses sapphire output windows that are doped with various other elements to tune their emission. In some cases, the UV emitted from the lamp during operation causes a blue glow from the window after the lamp is turned off. It is approximately the same color as Cherenkov radiation but is caused by simple phosphorescence.
Wafers of single-crystal sapphire are also used in the semiconductor industry as a substrate for the growth of devices based on gallium nitride (GaN), with a transparent conductive coating (TCC) formed from gallium nitride on a sapphire substrate. In order to account for the lattice mismatch between the GaN and the sapphire substrate, a nucleation layer is formed on the sapphire substrate. A mask, for example silicon dioxide (SiO2), is formed on top of the nucleation layer with a plurality of openings. GaN is then grown through the openings in the mask to form a lateral epitaxial overgrowth layer upon which defect-free GaN is then grown. The lateral epitaxial overgrowth compensates for the lattice mismatch between the sapphire substrate and the GaN. The use of a sapphire substrate eliminates the need for a cover glass and also significantly reduces the cost of the TCC, since such sapphire substrates are about one-seventh the cost of germanium substrates. Gallium arsenide on sapphire is commonly used in blue light-emitting diodes (LEDs).
The transparent conductive coating (TCC) may then be disposed on a gallium arsenide (GaAs) solar cell. In order to compensate for the lattice mismatches between the GaAs and the GaN, an indium gallium phosphate (InGaP) may be disposed between the GaAs solar cell and the GaN TCC to compensate for the lattice mismatch between the GaN and the GaAs. In order to further compensate for the lattice mismatch between the GaN and InGaP, the interface may be formed as a super lattice or as a graded layer. Alternatively, the interface between the GaN and the InGaP may be formed by the offset method or by wafer fusion. The TCC, in accordance with the present invention, is able to compensate for the lattice mismatches at the interfaces of the TCC while eliminating the need for a cover glass and a relatively expensive germanium substrate.
The theft of a sapphire known as the "Blue Water" is central to the plot of the novel Beau Geste by P. C. Wren and its various film adaptations.