Sapphire

Sapphire
SaphirSynthetique.jpg
General
Category Mineral Variety
Chemical formula aluminium oxide, Al2O3
Identification
Color Every color except red (which is ruby) or pinkish-orange (padparadscha)
Crystal habit massive and granular
Crystal system Hexagonal
Cleavage None
Fracture Conchoidal, splintery
Mohs Scale hardness 9.0
Luster Vitreous
Refractive index 1.762–1.778
Optical Properties Abbe number 72.2
Birefringence 0.008
Pleochroism Strong
Streak White
Specific gravity 3.95–4.03
Melting point 2030–2050 °C
Fusibility infusible
Solubility insoluble
Other Characteristics Coefficient of thermal expansion 5e−6–6.6e−6/K

Sapphire (from Greek sapphiros, originating from Ancient Greek hyacinthos) refers to gem varieties of the mineral corundum, an aluminium oxide (Al2O3), when it is a color other than red, in which case the gem would instead be a ruby. Trace amounts of other elements such as iron, titanium, or chromium can give corundum blue, yellow, pink, purple, orange, or greenish color. Pink-orange corundum are also sapphires, but are instead called padparadscha.

Because it is a gemstone, sapphire is commonly worn as jewelry. 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.

Contents

Natural sapphires

Sapphire is one of the two gem varieties of corundum, the other being the red ruby. Although blue is the most well known hue, sapphire is any color of corundum except red, red corundum known as ruby. Sapphire may also be colorless, and it also occurs in the non-spectral shades gray and black. Pinkish-orange sapphire is known as padparadscha.

The cost of natural sapphire varies depending on their color, clarity, size, cut, and overall quality as well as geographic origin. Significant sapphire deposits are found in Eastern Australia, Thailand, Sri Lanka, Madagascar, East Africa and in the United States at various locations (Gem Mountain) and in the Missouri River near Helena, Montana. [1] Sapphire and rubies are often found together in the same area, but one gem is usually more abundant.[2]

Blue sapphire

The 422.99-carat blue Logan sapphire

Color in gemstones breaks down into three components: hue, saturation, and tone. Hue is most commonly understood as the "color" of the gemstone. Saturation refers to the vividness or brightness or "colorfulness" of the hue, and tone is the lightness to darkness of the hue. [3] Blue sapphire exists in various mixtures of its primary and secondary hues, various tonal levels (shades) and at various levels of saturation (brightness): the primary hue must, of course, be blue.

Blue sapphire is evaluated based upon the purity of its primary hue. Purple, violet and green are the normal secondary hues found in blue sapphire. [4] Violet and purple can contribute to the overall beauty of the color, green is considered a distinct negative. [4] Blue sapphire with no more than 15% violet or purple should be considered fine color. [4] Blue sapphire with any amount of green as a secondary hue is not considered to be fine quality.[4] Gray is the normal saturation modifier or mask found in blue sapphire.[4] Gray reduces the saturation or brightness of the hue and therefore has a distinctly negative affect.

Fine blue sapphire can 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 a mixture of a green secondary hue or a gray mask. [3]

The 422.99-carat Logan sapphire in the National Museum of Natural History, Washington D.C. is one of the largest faceted gem-quality blue sapphires in the world.

Fancy color sapphire

Pink 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, sapphires of this color have appeared on the market as a result of a new treatment method called "lattice diffusion".

Padparadscha

Padparadscha is a pinkish-orange to orangy-pink coloured corundum, with a low to medium saturation and light tone, originally being mined in Sri Lanka, but also found in deposits in Vietnam and Africa. Padparadscha sapphires are very rare, and highly valued for their subtle blend of soft pink and orange hues. The name derives from the Sinhalese word for lotus blossom. Along with rubies they are the only corundums to be given their own name instead of being called a particular colored sapphire.

cut stone

The rarest of all padparadschas is the totally natural variety, with no beryllium or other treatment, and no heating. To find a stone that is certified by a reputable lab as being completely natural is extremely rare and the stone will be very expensive. High quality, unheated and untreated natural padparadscha sapphires will start off in the range of $5,000 per carat and rise by size, color, tone, cut, and clarity, to $20,000–30,000 per carat.

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

The 182 carat Star of Bombay 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[5]) 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. The 182 carat (36.4 g) Star of Bombay, housed in the National Museum of Natural History, Washington D.C., is a good example of a blue star sapphire.

Treatments

Sapphires may be treated by several methods to enhance and improve their clarity and color [6] It is common practice to heat natural sapphires to improve or enhance color. This is done by heating the sapphires to temperatures between 500 and 1800 °C for several hours, or by heating in a nitrogen-deficient atmosphere oven for seven days or more. Evidence of sapphire and other gemstones being subjected to heating goes back to, at least, Roman times. [7] Un-heated stones are quite rare and will often be sold accompanied by a certificate from an independent gemological laboratory attesting to "no evidence of heat treatment".

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 natural gem or one that has been enhanced by heat alone.

Treating stones with surface diffusion, however, is generally frowned upon; as stones chip or are repolished/refaceted the 'padparadscha' coloured layer can be removed. (There are some diffusion treated stones in which the color goes much deeper than the surface, however.) The problem lies in the fact that treated padparadschas are at times very difficult to detect, and they are the reason that getting a certificate from a reputable gemological lab (e.g. Gubelin, SSEF, AGTA, etc.) is recommended before investing in a padparadscha.

According to Federal Trade Commission guidelines, in the United States, disclosure of any mode of enhancement that has a significant effect on the gem's value.[8]

Mining

Sapphire from Madagascar

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). 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

Synthetic star sapphire

In 1902, French chemist Auguste Verneuil developed a process for growing synthetic sapphire crystals.[9] In the Verneuil process, fine alumina powder is added to an oxyhydrogen flame which is directed downward against a mantle.[10] 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. However the Verneuil process had the disadvantage that the crystals created with it had high internal strains. Many methods of manufacturing sapphire today are variations of the Czochralski process, invented in 1916.[11] 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.[12]

In 2003, the world's production of synthetic sapphire was 250 tons.(1.25 x 109carats).[12] 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.[12] 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.

Cermax xenon arc lamp with synthetic sapphire output window

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

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

  1. ↑ Wise, R. W. (2004). Secrets Of The Gem Trade, The Connoisseur's Guide To Precious Gemstones. Brunswick House Press. pp. 164–166. ISBN 097282238-0. 
  2. ↑ Wenk, Hans-Rudolf; Bulakh, A. G. (2004). Minerals: their constitution and origin. Cambridge, U.K.: Cambridge University Press. pp. 539–541. ISBN 0-521-52958-1. http://books.google.com/books?id=0GAvKQJ2JuwC&pg=RA1-PA539&vq=sapphire&source=gbs_search_s&sig=m_o9AxN7Wr0_Zmm7bCOFs7zsW2Y. 
  3. ↑ 3.0 3.1 Wise, R. W. (2004). Secrets Of The Gem Trade, The Connoisseur's Guide To Precious Gemstones. Brunswick House Press. pp. 18–22. ISBN 097282238-0. 
  4. ↑ 4.0 4.1 4.2 4.3 4.4 Wise, R. W. (2004). Secrets Of The Gem Trade, The Connoisseur's Guide To Precious Gemstones. Brunswick House Press. Chapter 22. ISBN 097282238-0. 
  5. ↑ Emsley, John (2001). Nature's Building Blocks: An A-Z Guide to the Elements. Oxford: Oxford University Press. pp. pp. 451 – 53. ISBN 0-19-850341-5. 
  6. ↑ Wise, R. W. (2004). Secrets Of The Gem Trade, The Connoisseur's Guide To Precious Gemstones. Brunswick House Press. pp. 169. ISBN 097282238-0. 
  7. ↑ Nassau, Kurt. Gemstone Enhancement. Butterworths. pp. 95. 
  8. ↑ Chapter I of Title 16 of the Code of Federal Regulations Part 23, Guides for Jewelry and Precious Metals and Pewter Industries
  9. ↑ M.A. Verneuil (September 1904) Memoire sur la reproduction artificielle du rubis per fusion, Annales de Chimie et de Physique
  10. ↑ Heaton, Neal; The production and identification of artificial precious stones in Annual Report of the Board of Regents of the Smithsonian Institution, 1911. USA: Government Printing Office. 1912. pp. p.217. http://books.google.com/books?id=hZIKAAAAIAAJ&pg=PA227. 
  11. ↑ Ramdeva, Ardamun (2004). "Synthetic Sapphires". Gemstones. JewelInfo4You. Retrieved on 2008-07-04.
  12. ↑ 12.0 12.1 12.2 Scheel, Hans Jr̲g; Fukuda, Tsuguo (2003) (PDF). Crystal growth technology. Chichester, West Sussex: J. Wiley. ISBN 0-471-49059-8. http://media.wiley.com/product_data/excerpt/98/04714905/0471490598.pdf. 

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