cover glass

Sapphire

[saf-ahyuhr]
Sapphire (antique greek: hyacinthos) refers to gem varieties of the mineral corundum, an aluminium oxide (Al2O3), when it is a color other than red (ruby) or pinkish-orange (padparadscha). Sapphire can be found naturally, or manufactured in large crystal boules. Because of its remarkable hardness, sapphire is used in many applications, including infrared optical components, watch crystals, high-durability windows, and wafers for the deposition of semiconductors, such as GaN nanorods and blue LEDs. The mineral corundum consists of pure aluminium oxide. Trace amounts of other elements such as iron, titanium, or chromium can give corundum blue, yellow, pink, purple, orange, or greenish color. Sapphire includes any gemstone-quality varieties of the mineral corundum except the fully saturated red variety, which is instead known as the ruby, and the pinkish-orange variety known as padparadscha.

Natural sapphire

Sapphire is one of the two gem varieties of the species corundum. Although blue is the best known hue, the gem occurs in virtually every spectral hue excepting red; red corundum is ruby. Sapphire may also be colorless, and it also occurs in the non-spectral shades gray and black.

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.

Blue sapphire

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.

Fancy color sapphire

Purple sapphires are lower in price than blue ones. These stones contain the trace element vanadium and come in a variety of shades. Yellow and green sapphires have traces of iron that gives them their color. Pink sapphires have a trace of the element chromium and the deeper the color pink the higher their monetary value as long as the color is going toward the red of rubies. Sapphires also occur in shades of orange and brown, and colorless sapphires are sometimes used as diamond substitutes in jewelry. Salmon-color padparadscha sapphires often fetch higher prices than many of even the finest blue sapphires. The word 'padparadscha' is Sinhalese for 'lotus flower'. Recently many sapphires of this color have appeared on the market as a result of a new treatment method called "lattice diffusion".

Color change sapphire

Color shift sapphires are blue in outdoor light and purple under incandescent indoor light. Color changes may also be pink in daylight to greenish under fluorescent light. Some stones shift color well and others only partially, in that some stones go from blue to bluish purple. Such color-change sapphires are widely sold as “lab” or “synthetic” alexandrite, which is accurately called an alexandrite simulant (also called alexandrium) since the latter is actually a type of chrysoberyl---an entirely different substance whose pleochroism is different and much more pronounced than color-change corundum (sapphire).

Star sapphire

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.

The Star of India (gem) is thought to be the largest star sapphire in the world and is currently on display at the American Museum of Natural History in New York City.

Treatments

Many sapphires are heat-treated or otherwise enhanced to improve their appearance and color, though some people object to such practices and prefer natural untreated stones. Heat treatments tend to improve the sapphire's color and clarity, and it is unusual to find un-heated stones for sale. Un-heated stones will usually be accompanied by a certificate attesting to "no evidence of heat treatment". Some sapphires, particularly pinks, are heated at low temperatures, and do not have evidence of heat treatment. Controversy has arisen as certain milky white sapphire stones, called geuda, can obtain clear blue color after heat treatment, dramatically increasing the appearance and value of an otherwise nearly worthless gem. Diffusion treatments are somewhat more controversial as they are used to add elements to the sapphire for the purpose of improving colors. Typically beryllium (Be) is diffused into a sapphire with very high heat, just below the melting point of the sapphire. Initially (c. 2000) orange sapphires were created with this process, although now the process has been advance and many colors of sapphire are often treated with beryllium. It is unethical to sell beryllium-treated sapphires without disclosure, and the price should be much lower than a gem that has not been Be-treated. The vast majority of sapphires offered to tourists in Bangkok are Be-treated.

Mining

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.

Synthetic sapphire

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.

Historical and cultural references

  • According to Rebbenu Bachya, and many English Bible translations, the word Sapir in the verse Exodus 28:18 means sapphire and was the stone on the Ephod representing the tribe of Issachar. Although it is true that the English word sapphire derives from the Hebrew sapir (via Greek sapphiros), this is extremely disputed. Sapphires were actually not known before the Roman Empire (and were initially considered to be forms of jacinth, rather than deserving of a word to themselves), and prior to that time sapphiros referred to blue gems in general. It is thought by scholars that the sapphire of the Bible was actually lapis lazuli - which was frequently sent as a gift between middle-eastern nations in Biblical times (Texas Natural Science Center, 2006). There is a wide range of views among traditional sources about which tribe the stone refers to.
  • Blue sapphire is associated with Saturn (Wojtilla, 1973), yellow sapphire with Jupiter in Vedic astrology. It is understood that word Sapphire seems to be a corrupted form of Sanipriya(Sanskrit:- Sani = Saturn, Priya = Beloved). Buddhist monks who moved to Middle East introduced the Stone as Sani piriya and eventually become sapir and sapphire.
  • Sapphire is the birthstone associated with September.
  • The 45th wedding anniversary is known as the sapphire anniversary.

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.

See also

References

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