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'Sapphire' is the non red variety of the mineral corundum, an aluminium oxide (Al₂O₃). It can be found naturally or manufactured in large crystal boules for varied applications, including infrared optical components, watch faces, high-durability windows, and wafers for the deposition of semiconductors such as GaN nanorods.
The mineral corundum consists of pure aluminium oxide. Trace amounts of other elements such as iron, titanium and chromium give corundum their 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 ruby, and the pinkish-orange variety known as padparadscha.

Contents

Natural sapphire

Blue sapphire

Fancy color sapphire

Color change sapphire

Star sapphire

Treatments

Mining

Synthetic sapphire

Historical and cultural references

References

Natural sapphire

Although blue is considered the ''normal'' color for sapphires, they can be found across a full range of spectral colors as well as brown, colorless, grey and black. Those other than blue in color are considered ''fancy color'' sapphires. Some natural sapphires can be found as completely transparent, or "white." White sapphires usually come out of the ground as light grey or brown and are then heated to make them clear. However, in very rare circumstances they will be found in a clear state.

Blue sapphire

Various shades of blue [dark and light] result from titanium and iron substitutions in the aluminium 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 are orangy-pink, pinkish-orange or pink-orange in color, which 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 "bulk 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 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) 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.

Treatments

Some 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. Diffusion treatments are somewhat more controversial as they are used to add elements to the sapphire for the purpose of improving color and thus enhancing the stone's value. Heat treatment is usually detectable by microscopic analysis, but diffusion isn't always detectable.

Mining

Sapphires are mined from alluvial deposits or from primary underground workings. The finest specimens are mined in Sri Lanka; both the Logan sapphire and the Star of Bombay originate from Sri Lankan mines. Sapphires are also mined in Madagascar, Thailand and Myanmar. Australia leads the world in sapphire production (as of 1987) specifically from basalt derived placer deposits in Queensland and New South Wales. Pakistan, Afghanistan, India, Tanzania and Kenya also produce sapphires. 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

Synthetic 'sapphire' crystals can be grown in cylindrical crystal boules of large size, up to many inches in diameter. As well as gemstone applications there are many other uses:
The first ever laser produced was based on the ''ruby'', the red variety of corundum. While this laser has few applications, the Ti-sapphire laser is popular due to the relatively rare ability to tune the laser wavelength in the red-to-near infrared region of the electromagnetic spectrum. It 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.
Pure sapphire boules can be sliced into wafers and polished to form transparent crystal slices. Such slices are used as watch faces in high quality watches, as the material's exceptional hardness makes the face resistant to scratching. Since sapphire ranks a 9 on the Mohs Scale, 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. Such surfaces often contain materials including silicon carbide, which, like diamond, are harder than sapphire and thus capable of causing scratches (Scheel 2003).
Sapphire is highly transparent at wavelengths of light between 170 nm to 5.3 $mu$m, as well as being five times stronger than glass. This leads to use of synthetic sapphire windows in high pressure chambers for spectroscopy.
Wafers of single crystal sapphire are also used in the semiconductor industry as a substrate for the growth of gallium nitride based devices.

One type of Xenon Arc Lamp known as Cermax (original brand name - generically known as a ceramic body xenon lamp) use 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.

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. However, this is extremely disputed as though it is true that the English word ''sapphire'' derives from the Hebrew ''sapir'' (via Greek ''sapphiros''), Sapphires were actually not really known about 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.

★ Sapphire is the birthstone associated with September.

★ The 45th wedding anniversary is known as the sapphire anniversary.

★ Sapphires are one of the four most valued stones. The other highly valued stones are rubies (sapphires that are red, caused by chromium impurities), emeralds, and diamonds.

References

★ Scheel, Hans J. and Tsuguo Fukuda, Eds, 2003, ''Crystal Growth Technology'', John Wiley & Sons ISBN 0-471-49059-8 (Available as PDF

★ Sofianides, Anna S. and George E. Harlow, 1997, ''Gems & Crystals'', Parkgate Books, pp. 44-55 ISBN 1-85585-391-4

★ Texas Natural Science Center, ''Mineral Lore and Mythology'', 2006

★ Mindat with location data

★ Wojtilla, G. Indian precious stones in the ancient East and West, Acta Orientakia (Budapest) 27, 2, 211-224.

★ http://www.gemstone.org/gem-by-gem/english/sapphire.html