Sapphire

For other uses, see Sapphire (disambiguation).

Sapphire
General
Category	Mineral Variety
Formula (repeating unit)	aluminium oxide, Al2O3
Crystal system	Trigonal (Hexagonal Scalenohedral) Symbol (-3 2/m) Space Group: R-3c
Identification
Color	Every color rock red (which is ruby) or pinkish-orange (padparadscha)
Crystal habit	massive and granular
Cleavage	None
Fracture	Conchoidal, splintery
Mohs scale hardness	9.0
Luster	Vitreous
Streak	White
Specific gravity	3.95–4.03
Optical properties	Abbe number 72.2
Refractive index	nω=1.768 - 1.772 nε=1.760 - 1.763, Birefringence 0.008
Pleochroism	Strong
Melting point	2030–2050 °C
Fusibility	infusible
Solubility	Insoluble
Other characteristics	CTE 5e−6 to 6.6e−6/K

Sapphire (Template:Lang-el) is a gemstone variety of the mineral corundum, an aluminium oxide (α-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.

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. Sapphire may also be colorless, and it also occurs in shades of gray and black. Pink-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. Sapphire and rubies are often found together in the same area, but one gem is usually more abundant.

Blue sapphire

The 423 carats (84.6 g) blue Logan sapphire

Crystal structure of 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. Blue sapphire exists in various mixtures of its primary (blue) and secondary hues, various tonal levels (shades) and at various levels of saturation (brightness).

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

The color of fine blue sapphires 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 admixture of a green secondary hue or a gray mask.

The 423 carats (84.6 g) 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.

Source of color

Red rubies are corundum which contain chromium impurities that absorb yellow-green light and result in deeper ruby red color with increasing content. Purple sapphires contain trace amounts of vanadium and come in a variety of shades. Corundum that contains ~0.01% of titanium is colorless. If trace amounts of iron are present, a very pale yellow to green color may be seen. If both titanium and iron impurities are present together, however, the result is a magnificent deep-blue color.

Unlike localized ("interatomic") absorption of light which causes color for chromium and vanadium impurities, blue color in sapphires comes from intervalence charge transfer, which is the transfer of an electron from one transition-metal ion to another via the conduction or valence band. The iron can take the form Fe²⁺ or Fe³⁺, while titanium generally takes the form Ti⁴⁺. If Fe²⁺ and Ti⁴⁺ ions are substituted for Al³⁺, localized areas of charge imbalance are created. An electron transfer from Fe²⁺ and Ti⁴⁺ can cause a change in the valence state of both. Because of the valence change there is a specific change in energy for the electron, and electromagnetic energy is absorbed. The wavelength of the energy absorbed corresponds to yellow light. When this light is subtracted from incident white light, the complementary color blue results. Sometimes when atomic spacing is different in different directions there is resulting blue-green dichroism.

Intervalence charge transfer is a process that produces a strong colored appearance at a low percentage of impurity. While at least 1% chromium must be present in corundum before the deep red ruby color is seen, sapphire blue is apparent with the presence of only 0.01% of titanium and iron.

Fancy color sapphire

Pink sapphire

Purple sapphires are lower in price than blue. Yellow and green sapphires are also commonly found. Pink sapphires deepen in color as the quantity of chromium increases. The deeper the pink color the higher their monetary value as long as the color is going towards 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-colored padparadscha (see below) sapphires often fetch higher prices than many of even the finest blue sapphires. Recently, sapphires of this color have appeared on the market as a result of a new treatment method called "lattice diffusion".

Padparadscha

Cut stone Padparadscha

Padparadscha is a pink-orange 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. 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. The rarest of all padparadschas is the totally natural variety, with no sign of treatment.

Color change sapphire

A rare variety of sapphire, known as color change sapphire, exhibits different colors in different light. Color change sapphires are blue in outdoor light and purple under incandescent indoor light; they 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. While color change sapphires come from a variety of locations, the gem gravels of Tanzania is the main source.

Certain synthetic color-change sapphires are 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 carats (36.4 g) 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) 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 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. It is common practice to heat natural sapphires to improve or enhance color. This is done by heating the sapphires in air to temperatures between 500 and 1800 °C for several hours, or by heating in a nitrogen-deficient atmosphere oven for seven days or more. Upon heating, the stone becomes a more blue in color but loses some of the silk. When high heat temperatures are used, the stone loses all of the silk and becomes clear under magnification. Evidence of sapphire and other gemstones being subjected to heating goes back to, at least, Roman times. 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 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 advanced 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 is generally frowned upon; as stones chip or are repolished/refaceted the 'padparadscha' colored 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 is required of any mode of enhancement that has a significant effect on the gem's value.

Mining

Sapphire from Madagascar

Sapphires are mined from alluvial deposits or from primary underground workings. The mining locations include Myanmar, Madagascar, Sri Lanka, Australia, Thailand, India, Pakistan, Afghanistan, Tanzania, Kenya and China. The Logan sapphire, the Star of India and the Star of Bombay originate from Sri Lankan mines. 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). In 1991 a new sapphire occurrence was discovered in Andranondambo, southern Madagascar. That area was industrially exploited since 1993 and has been almost abandoned few years later because of difficulties of exploiting sapphires in their bedrock. In the United States sapphires have been produced from deposits near Helena, Montana. Gem grade sapphires and rubies are also found in and around Franklin, North Carolina.

Synthetic sapphire

File:Labstarsapphirering.JPG

Synthetic star sapphire

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 downward 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. The disadvantage of Verneuil process is the grown crystals have high internal strains. 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.

In 2003, the world's production of synthetic sapphire was 250 tons (1.25 × 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 mode-locked. 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. Here glass is a layman term which refers not to the amorphous state, but to the transparency. Sapphire is not only highly transparent to wavelengths of light between 170 nm to 5.3 μm (the human eye can discern wavelengths from about 380 nm to 750 nm), but it is also five times stronger than glass and ranks a 9 on the Mohs Scale, and much tougher than tempered glass although not as much as synthetic stabilized zirconium oxide (such as yttria-stabilized zirconia). Along with zirconia and aluminum oxinitride, synthetic sapphire is used for shatter resistant windows in armored vehicles and various military body armour suits, in association with composites. Sapphire "glass" (although being crystalline) 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 and toughness makes it very resistant to scratching.

Cermax xenon arc lamp with synthetic sapphire output window

Synthetic sapphire is industrially produced from agglomerated aluminum oxide, sintered and fused in an inert atmosphere (hot isostatic pressing for example), yielding a transparent polycrystalline product, slightly porous, or with more traditional methods such as Verneuil, Czochralski, flux method etc, yielding a single crystal sapphire material which is non-porous and should be relieved of its internal stress.

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 allows for higher thermal loads and thus higher output power as compared to conventional Xe lamps with silica window.

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). The use of sapphire significantly reduces the cost, because it is about seven times cheaper than the alternative germanium. Gallium nitride on sapphire is commonly used in blue light-emitting diodes (LEDs).

Historical and cultural references

• Sapphire and ruby in ancient times being mined exclusively in Pakistan, Laos, India, Sri-Lanka and Afghanistan essentially, and quasi absent from the Middle East, some etymologists propose an old sanskrit origin to the semitic word 'sappir' : a dark colored precious stone called 'Sanipriya' , from 'sani' meaning saturn and 'priya', precious, i.e. "precious to Saturn"
• 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 has been stated that the English word sapphire derives from the Hebrew sapir (via Greek sapphiros), this is 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.
• Sapphire is one of the birthstones.
• The 45th wedding anniversary is known as the sapphire anniversary.

See also

• List of minerals
• Emerald
• Verneuil process

References

• Wise, R. W. (2004). Secrets Of The Gem Trade, The Connoisseur's Guide To Precious Gemstones. Brunswick House Press. ISBN 097282238-0.

General Resources

• Wojtilla, G. Indian precious stones in the ancient East and West, Acta Orientakia (Budapest) 27, 2, 211-224.
• Sofianides, Anna S. and George E. Harlow, 1997, Gems & Crystals, Parkgate Books, pp. 44–55 ISBN 1-85585-391-4
• Webmineral Corundum Page Webmineral with extensive crystallographic and mineralogical information on Corundum
• Farlang Sapphire References Historical resources on Sapphires, mining, notable gems etc.
• Mindat Sapphire page Mindat with extensive locality information
• ICA's Sapphire page International Colored Stone Sapphire page
• Macroscopic 10-Terabit–per–Square-Inch Arrays from Block Copolymers with Lateral Order Science magazine article about perspective usage of sapphire in digital storage media technology