Sapphire

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For other uses, see Sapphire (disambiguation).

Sapphire
The 423-carat (85 g) blue Logan sapphire
General
Category	Oxide mineral
Formula (repeating unit)	aluminium oxide, Al2O3
Identification
Color	Every color except a shade of red – which is called a ruby – or pinkish-orange (the padparadscha)
Crystal habit	massive and granular
Crystal system	Trigonal Symbol (32/m) Space Group: R3c
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	coefficient of thermal expansion (5.0–6.6)×10−6/K

Sapphire (Greek: σάπφειρος; sappheiros, 'blue stone',^[1] which probably referred instead at the time to lapis lazuli) is a gemstone variety of the mineral corundum, an aluminium oxide (α-Al₂O₃). Trace amounts of other elements such as iron, titanium, chromium, copper, or magnesium can give corundum blue, yellow, purple, orange, or a greenish color. Chromium impurities in corundum yield a pink or red tint, the latter being called a ruby.
Commonly, sapphires are worn in jewelry. Sapphires may be found naturally, by searching through certain sediments (due to their resistance to being eroded compared to softer stones) or rock formations. They also may be manufactured for industrial or decorative purposes in large crystal boules. Because of the remarkable hardness of sapphires, 9 on the Mohs scale (and of aluminium oxide in general), sapphires are used in some non-ornamental applications, including infrared optical components, such as in scientific instruments; high-durability windows; wristwatch crystals and movement bearings; and very thin electronic wafers, which are used as the insulating substrates of very special-purpose solid-state electronics (most of which are integrated circuits).

Contents 1 Natural sapphires 1.1 Blue sapphire 1.2 Fancy color sapphire 1.3 Padparadscha 1.4 Star sapphire 1.5 Color change sapphire 1.6 Source of color 1.7 Treatments 2 Mining 3 Synthetic sapphire 3.1 Transparent and tough 3.2 Common applications 3.3 Use as substrate for semiconducting circuits 4 Historical and cultural references 5 See also 6 References 7 External links

Natural sapphires

An uncut, rough yellow sapphire found at the Spokane Sapphire Mine near Helena, Montana

The sapphire is one of the three gem-varieties of corundum, the other two being ruby – defined as corundum in a shade of red—and padparadscha—a pinkish orange variety. Although blue is their most well-known color, sapphires may also be colorless and they are found in many colors including shades of gray and black.
The cost of natural sapphires varies depending on their color, clarity, size, cut, and overall quality – as well as their geographic origin. Significant sapphire deposits are found in Eastern Australia, Thailand, Sri Lanka, China (Shandong), Madagascar, East Africa, and in North America in a few locations, mostly in Montana.^[2] Sapphire and rubies are often found in the same geographic environment, but one of the gems is usually more abundant in any of the sites.^[3]

Blue sapphire

Teardrop cut blue 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 of the hue, and tone is the lightness to darkness of the hue.^[4] Blue sapphire exists in various mixtures of its primary (blue) and secondary hues, various tonal levels (shades) and at various levels of saturation (vividness).
Blue sapphires are evaluated based upon the purity of their primary hue. Purple, violet, and green are the most common secondary hues found in blue sapphires.^[5] Violet and purple can contribute to the overall beauty of the color, while green is considered to be distinctly negative. Blue sapphires with up to 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.^[5]
The color of fine blue sapphires may 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.^[4]
The 423-carat (85 g) Logan sapphire in the National Museum of Natural History, in Washington, D.C., is one of the largest faceted gem-quality blue sapphires in existence.

Fancy color sapphire

Pink sapphire

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 tending toward the red of rubies. In the United States, a minimum color saturation must be met to be called a ruby, otherwise the stone will be called a pink sapphire.^[6]
Sapphires also occur in shades of orange and brown. Colorless sapphires are sometimes used as diamond substitutes in jewelry. Natural padparadscha (pinkish orange) sapphires often draw higher prices than many of even the finest blue sapphires. Recently, more sapphires of this color have appeared on the market as a result of a new artificial treatment method that is called "lattice diffusion".^[7]

Padparadscha

Faceted padparadscha

Padparadscha is a delicate light to medium toned pink-orange to orange-pink hue corundum, originally found in Sri Lanka, but also found in deposits in Vietnam and parts of East Africa. Padparadscha sapphires are rare; the rarest of all is the totally natural variety, with no sign of artificial treatment.^[8]
The name is derived from the Sanskrit "padma raga" (padma = lotus; raga = color), a color akin to the lotus flower (Nelumbo nucifera ‘Speciosa’).^{[citation needed]}

Star sapphire

The Star of Bombay (182 carats (36 g))

A star sapphire is a type of sapphire that exhibits a star-like phenomenon known as asterism; red stones are known as "star rubies". Star sapphires contain intersecting needle-like inclusions following the underlying crystal structure that cause the appearance of a six-rayed "star"-shaped pattern when viewed with a single overhead light source. The inclusion is often the mineral rutile, a mineral composed primarily of titanium dioxide.^[9] The stones are cut en cabochon, typically with the center of the star near the top of the dome. Occasionally, twelve-rayed stars are found, or parallel whisker inclusions can produce a "cat's eye" effect.^[10]
The Black Star of Queensland, the largest gem-quality star sapphire in the world, weighs 733 carats.^[11] The Star of India (weighing 563.4 carats) is thought to be the second-largest star sapphire (the largest blue), and is currently on display at the American Museum of Natural History in New York City. The 182-carat Star of Bombay, located in the National Museum of Natural History, in Washington, D.C., is another example of a large blue star sapphire. The value of a star sapphire depends not only on the weight of the stone, but also the body color, visibility, and intensity of the asterism.

Color change sapphire

A rare variety of natural 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. 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 more accurately is called an alexandrite simulant (also called alexandrium) since the latter is a type of chrysoberyl—an entirely different substance whose pleochroism is different and much more pronounced than color-change corundum (sapphire). These stones are not sapphires.

Source of color

Crystal structure of sapphire

Sapphire ring made circa 1940

Rubies are corundum which contain chromium impurities that absorb yellow-green light and result in deeper ruby red color with increasing content.^[12] 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.^[13]
Unlike localized ("intra-atomic") 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.

Treatments

Sapphires may be treated by several methods to enhance and improve their clarity and color.^[14] It is common practice to heat natural sapphires to improve or enhance color. This is done by heating the sapphires in furnaces 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 more blue in color, but loses some of the rutile inclusions (silk). When high temperatures are used, the stone loses all silk (inclusions) and it becomes clear under magnification.^[15] The inclusions in natural stones are easily seen with a jeweler's loupe. Evidence of sapphire and other gemstones being subjected to heating goes back at least to Roman times.^[16] Un-heated natural stones are somewhat rare and will often be sold accompanied by a certificate from an independent gemological laboratory attesting to "no evidence of heat treatment".

Yogo sapphire

Yogo sapphires sometimes do not need heat treating because their cornflower blue coloring is uniform and deep, they are generally free of the characteristic inclusions, and they have high uniform clarity.^[17] When Intergem Limited began marketing the Yogo in the 1980s as the world's only guaranteed untreated sapphire, heat treatment was not commonly disclosed; by 1982 the heat treatment became a major issue.^[18] At that time, 95% of all the world's sapphires were being heated to enhance their natural color.^[19] Intergem's marketing of guaranteed untreated Yogos set them against many in the gem industry. This issue appeared as a front page story in the Wall Street Journal on August 29, 1984 in an article by Bill Richards, Carats and Schticks: Sapphire Marketer Upsets The Gem Industry.^[19]
Diffusion treatments are used to add impurities to the sapphire to enhance color. Typically beryllium is diffused into a sapphire under very high heat, just below the melting point of the sapphire. Initially (c. 2000) orange sapphires were created, although now the process has been advanced and many colors of sapphire are often treated with beryllium. The colored layer can be removed when stones chip or are repolished or refaceted, depending on the depth of the impurity layer. Treated padparadschas may be very difficult to detect, and many stones are certified by gemological labs (e.g., Gubelin, SSEF, AGTA).
According to United States Federal Trade Commission guidelines, disclosure is required of any mode of enhancement that has a significant effect on the gem's value.^[20]

Mining

Sapphire from Madagascar

Sapphires are mined from alluvial deposits or from primary underground workings. Commercial mining locations for sapphire and ruby include (but are not limited to) the following countries: Afghanistan, Australia, Myanmar/Burma, Cambodia, China, Colombia, India, Kenya, Laos, Madagascar, Malawi, Nepal, Nigeria, Pakistan, Sri Lanka, Tajikistan, Tanzania, Thailand, USA, and Vietnam.^[21] The Logan sapphire, the Star of India, and the Star of Bombay originate from Sri Lankan mines. Madagascar is the world leader in sapphire production (as of 2007) specifically its deposits in and around the town of Ilakaka.^[22] Prior to the opening of the Ilakaka mines, Australia was the largest producer of sapphires (such as in 1987).^[23] In 1991 a new source of sapphires was discovered in Andranondambo, southern Madagascar. That area has been exploited for its sapphires started in 1993, but it was practically abandoned just a few years later – because of the difficulties in recovering sapphires in their bedrock.^[24]
In North America, sapphires have been mined mostly from deposits in Montana: fancies along the Missouri River near Helena, Montana, Dry Cottonwood Creek near Missoula, Montana, and Rock Creek near Philipsburg, Montana. Fine blue Yogo sapphires are found at Yogo Gulch west of Lewistown, Montana.^[25] A few gem-grade sapphires and rubies have also been found in the area of Franklin, North Carolina.^{[citation needed]}
The sapphire deposits of Kashmir are still well known in the gem industry,^[26] despite the fact that the peak production from this area mostly took place in a relatively short period at the end of the nineteenth and early twentieth centuries.^[27] At present, the world record price-per-carat for a sapphire at auction is held by a sapphire from Kashmir, which sold for more than $145,000 per carat (more than $3.8 million dollars in total) in November 2011.^[28]

Synthetic sapphire

Synthetic star sapphire

Synthetic sapphire

In 1902 the French chemist Auguste Verneuil developed a process for producing synthetic sapphire crystals.^[29] In the Verneuil process, named for him, fine alumina powder is added to an oxyhydrogen flame, and this is directed downward against a mantle.^[30] The alumina in the flame is slowly deposited, creating a teardrop shaped "boule" of sapphire material. Chemical dopants can be added to create artificial versions of the ruby, and all the other natural colors of sapphire, and in addition, other colors never seen in geological samples. Artificial sapphire material is identical to natural sapphire, except it can be made without the flaws that are found in natural stones. The disadvantage of Verneuil process is that the grown crystals have high internal strains. Many methods of manufacturing sapphire today are variations of the Czochralski process, which was invented in 1916.^[31] In this process a tiny sapphire seed crystal is dipped into a crucible made of the precious metal iridium or molybdenum,^[32] containing molten alumina, and then slowly withdrawn upward at a rate of one 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.^[33]
Synthetic sapphire is industrially produced from agglomerated aluminium 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.
In 2003 the world's production of synthetic sapphire was 250 tons (1.25 × 10⁹ carats), mostly by the United States and Russia.^[33][34] 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 their relatively rare capacity to be tuned to various wavelengths in the red and 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.

Transparent and tough

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 150 nm (UV) and 5500 nm (IR) (the human eye can discern wavelengths from about 380 nm to 750 nm^[35]), but is also extraordinarily scratch-resistant (compare sapphire's scratch hardness of 400 versus 50 for window glass and 100 for quartz.^[36])

Common applications

Along with zirconia and aluminium oxynitride, synthetic sapphire is used for shatter resistant windows in armored vehicles and various military body armor suits, in association with composites.
A common use of synthetic sapphire is in sapphire optical windows. The key benefits of sapphire windows are:

• Very wide optical transmission band from UV to near-infrared, (0.15-5.5 µm)
• Significantly stronger than other optical materials or standard glass windows
• The hardest natural substance next to diamond
• Highly resistant to scratching and abrasion (9 Mohs scale)
• Extremely high melt temperature (2030°C)
• Totally unaffected by all chemicals except some very hot caustics

Sapphire glass windows (although being crystalline) are made from pure sapphire boules that have been grown in an application specific crystal orientation, typically along the optical axis, the c-axis, for standard optical windows for minimum birefringence. The boules are sliced up into the desired window thickness and finally polished to the desired surface finish. Sapphire optical windows can be polished to a wide range surface finishes due to its crystal structure and it hardness. The surface finishes of Optical Windows are normally called out by the Scratch-Dig specifications in accordance with the globally adopted MIL-O-13830 specification.
Sapphire glass windows are used in high pressure chambers for spectroscopy, crystals in various watches, and windows in grocery store barcode scanners since the material's exceptional hardness and toughness makes it very resistant to scratching.^[33]

Cermax xenon arc lamp with synthetic sapphire output window

One type of xenon arc lamp (originally called the "Cermax" its first brand name), which is now known generically as the "ceramic body xenon lamp", uses sapphire crystal output windows that tolerate higher thermal loads – and thus higher output powers when compared with conventional Xe lamps with pure silica window.^[37]

Use as substrate for semiconducting circuits

Thin sapphire wafers also are used as an insulating substrate in high-power, high-frequency CMOS integrated circuits. This type of IC is called a silicon on sapphire or "SOS" chip. These are especially useful for high-power radio-frequency (RF) applications such as those found in cellular telephones, police car and fire truck radios, and satellite communication systems. "SOS" allows for the monolithic integration of both digital and analog circuitry all on one IC chip.
The reason for choosing wafers of artificial sapphire, rather than some other substance, for these substrates is that sapphire has a quite low conductivity for electricity, but a much-higher conductivity for heat. Thus, sapphire provides good electrical insulation, while at the same time doing a good job at helping to conduct away the significant heat that is generated in all operating integrated circuits.
Once the single crystal sapphire boules are grown they are cored-sliced into cylindrical pieces. Wafers are then sliced from these cylindrical cores. These wafers of single-crystal sapphire material are also used in the semiconductor industry as a non-conducting substrate for the growth of devices based on gallium nitride (GaN). The use of the sapphire material significantly reduces the cost, because this has about one-seventh the cost of germanium. Gallium nitride on sapphire is commonly used in blue light-emitting diodes (LEDs).^[38]

Historical and cultural references

• Etymologically, the English word “sapphire” derives from Latin sapphirus, sappirus from Greek σαπφειρος (sappheiros) from Hebrew סַפִּיר (sappir) from Old Iranian sani-prijam, from Sanskrit, Shanipriya (शनिप्रिय), from "shani" (शनि) meaning "Saturn" and "priya" (प्रिय), dear, i.e. literally “dear to Saturn”.^[1]
• The Greek term for sapphire quite likely was misapplied, and instead, used to refer to lapis lazuli.^[1]
• During the Medieval Ages, European lapidaries came to refer to blue corundum crystal by its “sapphire-blue” color, whence the modern name for “sapphire”.^{[citation needed]}