Everything about Sapphire totally explained
Sapphire refers to gem varieties of the mineral
corundum, an
aluminium oxide (Al
2O
3), when it's a color other than red. 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.
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.
Natural sapphire
Sapphires and rubies are formed at great depth in the earth's mantle or the lowest part of the crust. 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 that'll be found in a clear state.
Blue sapphire
Various shades of blue [darkand light] result from
titanium and
iron substitutions in the aluminium oxide
crystal lattice. Some stones are not well saturated and show tones of grey. 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 wasn't heated; if the
silk isn't 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 grey 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.
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's unusual to find un-heated stones for sale. Diffusion treatments are somewhat more controversial as they're used to add elements to the sapphire for the purpose of improving colors.
Mining
Sapphires are mined from
alluvial deposits or from primary underground workings. The finest specimens are mined in
Sri Lanka and the disputed territory of Kashmir. Both the
Logan sapphire and the
Star of Bombay originate from Sri Lankan mines. Sapphires are also mined in
Australia,
Madagascar,
Thailand and
Myanmar. 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. 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 for high quality
watch crystals, 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 μm, 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), 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.
A transparent conductive coating (TCC) formed from gallium nitride GaN 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 grown through the openings in the mask to form a lateral epitaxial overgrowth layer upon which defect-free GaN is 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 the cost of germanium substrates. The TCC may then be disposed on a 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. However, this is extremely disputed as though it's true that the English word sapphire derives from the Hebrew sapir (via Greek sapphiros), 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.
Further Information
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