What is Corundum and What are its Basic Qualities?
Corundum is an aluminum oxide that commonly forms hexagonal barrel-shaped prisms that taper at both ends or as thin tabular hexagonal plates. It has a hardness of 9 on the Mohs scale, making it one of the most durable commercial gemstones. It has no dominant cleavage and fractures in a conchoidal manner. A high density of ~4.0 g/cm3 (most silicate minerals are ~2.6 g/cm3) results in corundum occurring in secondary placer deposits and recoverable by panning methods, similar to how you would recover placer gold.
Corundum comes in all colours of the rainbow but is most commonly found as opaque crystals with dull colours. Red corundum is called ruby, blue corundum is called sapphire, and all other colours are called fancy sapphires. Some varieties of corundum will fluoresce under short wave and long wave UV light if there is enough chromium in the crystal structure but little iron, which tends to quench any emitted energy.
Pure corundum is colourless and clear if transparent or pale white if opaque. This mineral also has low dispersion so the value of the stones comes not from fire generated (as in diamond), but rather from the intensity of colours seen. The vivid colours of corundum gem varieties, such as ruby and sapphire, arise primarily from elemental substitution in the Al site by transition metal elements. The most common cations to substitute are Fe+2, Fe+3, Ti+4, Cr+3, and V+3.
A continuum of colour saturation exists between pink sapphire and ruby that is correlated with trace amounts of Cr. There is no official cutoff for the amount of Cr needed for ruby, but usually rubies will have up to ~1 wt% of Cr2O3. When Cr substitutes for Al, wide absorption bands are generated in the violet (~450 nm) and green-yellow (~500 nm) ranges, as well as overlap a bit into the blue region. The red region of the electromagnetic spectrum (~650 nm) does not have very much absorption at all and results in all colours but red being blocked by ruby.
But there is another trick up ruby's sleeve that makes its red almost jump out at the observer. When Cr is introduced into corundum it makes the mineral fluorescent under UV light. This means that UV energy from normal light is accepted into the crystal and then re-emitted at a lower energy level - conveniently in the red region, thus amplifying the intensity of red in ruby under daylight conditions. However, if any iron is present it will usually absorb the red fluorescence from UV light. Thus, the finest rubies are those that have little to no iron in their crystal structure.
Blue sapphires are generated primarily from pairs of Fe+2 and Ti+4 substituting into the crystal structure for Al+3. The process of intervalence charge transfer (essentially continual swapping of electrons, bouncing back and forth) occurs between the Fe and Ti and all colours except blue are absorbed. So like ruby, it is the absorption of all other colours from full spectrum light (aka white light) that generates the beautiful blues in sapphires, rather than the "generation" of the blue colour. Very small amounts of these elements (only ~0.01 wt% Fe and Ti) are needed to produce the vivid blues.
Other colours are generated from a combination of these elements, as well as other minor cations and defects in the crystals. Also, a single corundum gemstone can be multi-coloured from different concentrations of metals in different parts of the crystal - this is called zoning.
Some sapphires also show an optical characteristic called asterism, which is most commonly seen as a six or twelve pointed star. These "arms" of the star are generated from oriented inclusions of long and skinny minerals (almost always the mineral rutile, a titanium oxide, TiO2). Specimens found with these inclusions are often cut and polished in a rounded and polished cabochon style to emphasize the nature of this optical effect. Rutile inclusions can occur in both sapphires and rubies, although it is more common in sapphires.
As mentioned previously, most euhedral corundum crystals show a hexagonal growth habit forming squat plates or tapering "barrels". Often the crystals will show a modified growth habit or, if found in placers, will have their delicate corners worn away. The images below show this habit with varying degrees of modification.
These two samples from Greenland occur in different geological environments which is easily seen by the different colours and textures of the "host" rocks. The rubies in the lower dark brown rock are the classic tapered barrel shape, and those in the upper white and black rock display a more unconventional blocky habit. Photo courtesy of True North Gems.
Close up of the above image showing the tapered barrel shape of these naturally coloured ruby crystals. The blue minerals around the rubies are kyanite. Photo courtesy of True North Gems.
The Kitaa Ruby from Greenland is possibly the largest ruby ever found in the Northern Hemisphere. This raw specimen, prior to carving, is an aggregate of intergrown ruby crystals. Photo courtesy of True North Gems.
This large pink sapphire sample from Greenland shows a rough hexagonal outline of the parent mineral, corundum. Photo courtesy of True North Gems.
This large raw sapphire from Baffin Island, Nunavut shows an elongated tapered barrel shape. The smaller faceted sapphire (bottom) is untreated and from the same location. Photo courtesy of True North Gems.
This ruby crystal shows a rough hexagonal outline with a squat crystal form. Note the natural striations on the crystal face with intersections at ~120 degrees and the natural lichen still clinging to the stone.
This is the same specimen as above, but shown under shortwave UV light. Note the red fluorescence of the ruby and white-blue fluorescence of the lichen.
In displaying the stone's vibrancy, the cut of sapphires and rubies is not as critical as that for diamond. However, a well planned cut will always make a stone exhibit the best possible colours and decrease the distracting presence of any inclusions. Light-coloured stones are often cut deeper to intensify colours. Deeply-coloured stones are often cut shallower in order to soften the colour. Stones that are cut too shallow display "windows" through the stone because much of the light entering will not be reflected back to the observer. If colour zoning in a specimen is present, a proper cut can hide the zoning, or if desired, emphasize it.
These polished rubies and pink sapphires from Greenland are untreated. Photo courtesy of True North Gems.
This vibrant pink sapphire from Greenland weighs 0.71 carats. Photo courtesy of True North Gems.
These rubies and pink sapphires have an assortment of cuts and range in weight from 0.22 to 5.70 carats. Photo courtesy of True North Gems.
The Kitaa Ruby from Greenland was carved from a large piece of rough ruby (88 grams, or 440 carats; shown in the previous page). After carving, it weighed a total of 302 carats. This is the front side of the carving. Photo courtesy of True North Gems.
The back side of the carved Kitaa Ruby. Photo courtesy of True North Gems.
The rough and two carved sides of the Kitaa Ruby. Photo courtesy of True North Gems.
The Carmen Lucia ruby, a spectacular 23.1 carat stone from Burma, is well cut and set in platinum with two diamonds. A ruby of this size and quality is extremely rare. This particular stone (and ring) is housed in the Smithsonian Institute's Janet Annenberg Hooker Hall of Geology, Gems and Minerals. Note the natural "fluid-filled" inclusion roughly in the center of this stone. Photo courtesy of the Smithsonian Institute's National Museum of Natural History.
Carmen Lucia ruby, side view. Note how the fluid filled inclusion is invisible from this angle. Photo courtesy of the Smithsonian Institute's National Museum of Natural History.
Three deeply-coloured sapphires from Baffin Island, Nunavut. Photo courtesy of True North Gems.
Cushion Cut blue sapphire with good colour and clarity. Photo courtesy of the Gemological Institute of America.
Rutile fibers (also known as "silk") in a faceted blue sapphire. These inclusions are what give rise to asterism in gemstones, which are often cut en cabochon to emphasize their star effect. Photo courtesy of the Gemological Institute of America.
These two fancy yellow sapphires from Baffin Island, Nunavut originated from the same deposit as the deeply-coloured sapphires shown previously. Photo courtesy of True North Gems.
This black star sapphire shows typical asterism of corundum as a result of rutile (TiO2) inclusions.
Corundum's hardness of 9 is distinct for this mineral, as is its hexagonal nature. However, both of these properties are more useful in the field than in the jewellery store when it is actually OK to scratch the stone! Because corundum only occurs in specific rock types with specific other minerals, the geological setting can often rule out, or strengthen, the possibility of sapphire or ruby. When cut, important qualities of corundum for identification are its mineral and fluid inclusions, low dispersion, high density, and two refractive indices (it is dichroic).
Sapphires are sometimes confused with spinel, kyanite, benitoite, and tanzanite. Rubies are sometimes confused with garnet, tourmaline, and spinel. All of these stones are further described in your textbook, and we'll learn more about tanzanite, garnet, and tourmaline later in the course.
Rubies and sapphires are valued primarily for their colour, then for their clarity. Since rubies and sapphires are treasured for their intense colour it is no surprise that this is the primary deciding factor for its value. A nice clean stone is more attractive than one that is heavily included. Origin has a strong impact on the value of stones; size and cut are also important.
Stones originating from conflict zones or undisclosed locations are often undesirable to many consumers. Conversely, corundum sourced from historical locations are, in a sense, analogous to brand name items like Gucci or Armani. When considering size, large stones are rarer. For cut, a poorly faceted stone will not show its best colours and will likely have to be re-cut and therefore lose some carat-weight. So, like diamonds, there are 4+1 Cs to evaluating gem corundum: colour, clarity, cut, carat… plus Country of origin!
The best sapphires are valued according to the purity and intensity of the blue, with the "ideals" showing either refreshing cornflower blue or a velvety royal blue. Most of the highest calibre sapphires come from three different regions of the world. Stones from Kashmir are often the most valuable and exhibit an intense, velvet-like blue. Burmese sapphires are also highly valued for their saturated blue. Burmese stones often show wonderful asterisms. Finally, Sri Lanka and its cornflower blues, not to mention their very large sizes, are also prized.
This photo shows a blue-violet example of the cornflower weed, the Centaurea cyanus. Photo courtesy of the Smithsonian Institute's National Museum of Natural History.
Rubies with Pigeon's Blood red colouration, a colour described as a pure red with a hint of blue, are highly valued. Pigeon's Blood red rubies typically originate from Burma, but other noteworthy localities of high quality ruby are found in Vietnam, Sri Lanka, and Thailand. The highest quality rubies will show a strong red fluorescence and sometimes contain fine rutile silk that scatters the light across the stone, displaying a full bodied colour. Rubies with the finest optical qualities (colour, clarity) rarely have significant weights and a stone of ~2 carats is considered quite large.
The fancy sapphires (any colour of corundum other than blue or red) are more volatile in value and are driven by the consumer market place. For instance, fancy hot pink sapphires spiked in value over the last ~5-10 years whereas colourless, yellow, green, and orange stones have not received as much attention from consumers. One exception to this are Padparadscha sapphires that have an orange-pink colouration. These stones are known to have prices that approach the levels of fine rubies.
Once examined by a gemologist, rubies and sapphires will be ascribed a rating based on the 4+1 Cs. Ratings for coloured stones are less comprehensive than that for diamonds, and the five usual categories used are Poor, Fair, Good, Very Good, and Exceptional. In most jewellery stores, the top stones will be of "Good" quality. Very Good stones are found in high end stores, and Exceptional stones are found only in exceptional circumstances. The following tables are estimates of prices per carat for "Good" rubies and sapphires from "non-prime sources" based on size (prices accurate as of ~2007). Note how the value per carat increases with increasing size.
|Less than 0.5||$25 to $350|
|0.5 to 1.0||$350 to $600|
|1.0 to 2.0||$600 to $2500|
|2.0 to 5.0||$2500 to $4500|
|More than 5.0||$4500 to $8000|
|Less than 0.5||$175 to $200|
|0.5 to 1.0||$200 to $350|
|1.0 to 2.0||$350 to $600|
|2.0 to 5.0||$600 to $1000|
|More than 5.0||$1000 to $2500|
Record setting prices for rubies and sapphires rival prices (on a per carat basis) of the finest diamonds. A fine Burmese ruby weighing 8.62 carats and set in a gold Bulgari ring sold at a Christie's auction in 2006 for $3.6 million USD. That works out to about $425,000 per carat! An interesting quote by J.B. Tavernier (a famous historical gem trader) written in 1676 was included in the item's description for the auction. It still holds true today, almost four hundred years later:
"When a ruby exceeds 5 carats, and is perfect, it is sold for whatever is asked for it."
In 1993 a much larger ruby (38.12 carats, loose) was sold at auction for $5.9 million USD, at approximately $150,000 per carat.
Of recently sold fine sapphires, a 42.28 carat Kashmir sapphire was auctioned by Christie's in 2008 for $3.5 million USD, a per carat price of approximately $82,000. A smaller (22.66 carats) but finer stone fetched about $3 million USD in 2007, which is approximately $135,000 per carat. A fine 10.14 carat Padparadscha sapphire was sold in 2004 for $250,000 USD, or approximately $25,000 per carat.
For more information, Richard Hughes' website on sapphire and rubies provides an excellent and in depth look at evaluating rubies and sapphires. It contains stunning photographs. Christie's Fine Art Auctions is also a fun place to search for valuable gemstones and track down record breaking prices for fine and exceptional stones.
At any gem corundum mine, most of the material found is not of gem quality. As a consequence, much effort has been directed to improving the quality of mined stones ever since mining of corundum began. Almost all (~99%) sapphires and rubies are heat treated to change colours, intensify them, and increase clarity.
The robust nature of corundum and the mineralogical changes that occur when heat treating corundum are quite propitious. The solidinclusions that detract from a stone's clarity (although some, including me, would argue that these add character to a stone) are usually comprised of elements that, coincidently, can be incorporated into corundum's crystal structure. These inclusions are commonly rutile (TiO2), spinel (ideally MgAl2O4, but often "impure"), and iron titanium oxides such as ilmenite (FeTiO3). Corundum's melting point (~2000 °C) is higher than most of its common inclusions. Thus, heating allows the solid inclusions to resorb or "melt" back into the corundum's crystal structure without melting the corundum.
Heating improves clarity by "removing" the opaque inclusions, but also by allowing chromophore-type elements, such as Ti and Fe, to become part of the corundum crystal and help colour the stone. Under the right conditions, Fe can be chemically "persuaded" to acquire a charge of either +2 or +3 which will also affect the resulting colour. Fluid inclusions and fracture-type inclusions won't add to a stone during heat treatment, but these features can be annealed or healed to make them "disappear". Consequently, the clarity of the treated stone can increase dramatically.
Corundum also commonly undergoes diffusion treatment where an element not associated with the crystal is forced into the structure via heat, pressure, and chemical gradients. This allows the "treater" or chemist, to impart a variety of colours to the original crystal. Diffusion is most commonly used to change colourless sapphires into Padparadscha sapphires with the diffusion of beryllium (Be).
Treatment to corundum is typically done to rough stones that have not yet been cut since exposure to the high heat can also cause new fractures to form. Otherwise, a faceted stone could lose considerable value if it broke during the heat treatment process.
Due to its simple chemical makeup, corundum has been produced synthetically since 1837 and gem quality synthetic corundum entered the marketplace in the early 1900's. All colours can be produced synthetically, and very large sizes (more than 100 carats!) can also be achieved using Czochralski's Drawing Method.
This process involves taking the necessary oxide components for gem corundum (e.g., Al2O3, Fe2O3, TiO2, Cr2O3) in powdered form and melting them together in a hot container that is just barely over the crystallizing temperature. A rod with a small corundum seed crystal is lowered into the molten material and then very slowly removed. As the crystal is raised above the nutrient rich molten mixture a small amount of corundum is formed at the interface between the seed crystal and the molten mixture. As the rod is slowly pulled upwards, new corundum continually grows below!
Another common technique for growing synthetic corundum is called the Vernueil Process and involves "dripping" of melted corundum onto bulb shaped corundum crystal. This process is similar to how stalactites form in caves.
Luckily, even the best synthetic corundum crystals will show signs of their history by specific identifiable (but often only to trained gemologists) inclusions related to their growth. Synthetic stones, as expected, are considerably cheaper than natural stones.
Imitation sapphires and rubies have always been around, often times by accident. Prior to robust testing and mineralogical identification, many spinels and garnets were actually thought to be rubies. Mistaken identifications have also been made with sapphires, the most common being kyanite, blue glass, and topaz. As expert identification became more commonplace and prominent in the 1800's, these "misidentified" stones migrated to the "imitation" category when being sold as rubies or sapphires. Today, most of these stones are considered gem varieties in their own right.
Today, imitations for sapphires and rubies are mostly in the form of doublets where natural colourless ruby or sapphire has a coloured synthetic corundum crystal glued underneath. Other composite constructions also exist and can be difficult to identify even with the proper gemological tools.
Not to be confused with Carborundum.
|Category||Oxide mineral – Hematite group|
|Aluminium oxide, Al|
|Crystal class||Hexagonal scalenohedral (3m)|
H-M symbol: (3 2/m)
|Space group||R3c (No. 167)|
|Unit cell||a = 4.75 Å, c = 12.982 Å; Z = 6|
|Color||Colorless, gray, golden-brown, brown; purple, pink to red, orange, yellow, green, blue, violet; may be color zoned, asteriated mainly grey and brown|
|Crystal habit||Steep bipyramidal, tabular, prismatic, rhombohedral crystals, massive or granular|
|Twinning||Polysynthetic twinning common|
|Cleavage||None – parting in 3 directions|
|Fracture||Conchoidal to uneven|
|Mohs scalehardness||9 (defining mineral)|
|Luster||Adamantine to vitreous|
|Diaphaneity||Transparent, translucent to opaque|
|Optical properties||Uniaxial (−)|
|Refractive index||nω = 1.767–1.772|
nε = 1.759–1.763
|Melting point||2,044 °C (3,711 °F)|
|Alters to||May alter to mica on surfaces causing a decrease in hardness|
|Other characteristics||May fluoresce or phosphoresce under UV light|
|Sapphire||Any color except red|
|Emery||Black granular corundum intimately mixed with magnetite, hematite, or hercynite|
Corundum is a crystalline form of aluminium oxide (Al
3) typically containing traces of iron, titanium, vanadium and chromium. It is a rock-forming mineral. It is a naturally transparent material, but can have different colors depending on the presence of transition metal impurities in its crystalline structure. Corundum has two primary gem varieties: ruby and sapphire. Rubies are red due to the presence of chromium, and sapphires exhibit a range of colors depending on what transition metal is present. A rare type of sapphire, padparadscha sapphire, is pink-orange.
The name "corundum" is derived from the Tamil-Dravidian word kurundam (ruby-sapphire) (appearing in Sanskrit as kuruvinda).
Because of corundum's hardness (pure corundum is defined to have 9.0 on the Mohs scale), it can scratch almost all other minerals. It is commonly used as an abrasive on sandpaper and on large tools used in machining metals, plastics, and wood. Emery, a variety of corundum with no value as a gemstone, is commonly used as an abrasive. Emery's blackish hue is caused by impurities of hematite, spinel, or magnetite.
In addition to its hardness, corundum has a density of 4.02 g/cm3 (251 lb/cu ft), which is unusually high for a transparent mineral composed of the low-atomic mass elements aluminium and oxygen.
Geology and occurrence
Corundum occurs as a mineral in mica schist, gneiss, and some marbles in metamorphicterranes. It also occurs in low-silica igneoussyenite and nepheline syeniteintrusives. Other occurrences are as masses adjacent to ultramafic intrusives, associated with lamprophyredikes and as large crystals in pegmatites. It commonly occurs as a detrital mineral in stream and beach sands because of its hardness and resistance to weathering. The largest documented single crystal of corundum measured about 65 cm × 40 cm × 40 cm (26 in × 16 in × 16 in), and weighed 152 kg (335 lb). The record has since been surpassed by certain synthetic boules.
Corundum for abrasives is mined in Zimbabwe, Pakistan, Afghanistan, Russia, Sri Lanka, and India. Historically it was mined from deposits associated with dunites in North Carolina, US, and from a nepheline syenite in Craigmont, Ontario.Emery-grade corundum is found on the Greek island of Naxos and near Peekskill, New York, US. Abrasive corundum is synthetically manufactured from bauxite.
Four corundum axes dating to 2500 BC from the Liangzhou culture have been discovered in China.
The Verneuil process allows the production of flawless single-crystal sapphire and ruby gems of much larger size than normally found in nature. It is also possible to grow gem-quality synthetic corundum by flux-growth and hydrothermal synthesis. Because of the simplicity of the methods involved in corundum synthesis, large quantities of these crystals have become available on the market at a fraction of the cost of natural stones.
Apart from ornamental uses, synthetic corundum is also used to produce mechanical parts (tubes, rods, bearings, and other machined parts), scratch-resistant optics, scratch-resistant watch crystals, instrument windows for satellites and spacecraft (because of its transparency in the ultraviolet to infrared range), and laser components. For example, the KAGRA gravitational wave detector's main mirrors are 23 kg (50 lb) sapphires, and Advanced LIGO considered 40 kg (88 lb) sapphire mirrors. Corundum has also found use in the development of ceramic armour thanks to its high hardiness.
Structure and physical properties
Corundum crystallizes with trigonal symmetry in the space group R3c and has the lattice parameters a = 4.75 Å and c = 12.982 Å at standard conditions. The unit cell contains six formula units.
The toughness of corundum is sensitive to surface roughness and crystallographic orientation. It may be 6–7 MPa·m1/2 for synthetic crystals, and around 4 MPa·m1/2 for natural.
In the lattice of corundum, the oxygen atoms form a slightly distorted hexagonal close packing, in which two-thirds of the octahedral sites between the oxygen ions are occupied by aluminium ions. The absence of aluminium ions from one of the three sites breaks the symmetry of the hexagonal close packing, reducing the space group symmetry to R3c and the crystal class to trigonal. The structure of corundum is sometimes described as a pseudohexagonal structure.
|Wikimedia Commons has media related to Corundum.|
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- ^ abGiuliani, Gaston; Ohnenstetter, Daniel; Fallick, Anthony E.; Groat, Lee; Fagan; Andrew J. (2014). "The Geology and Genesis of Gem Corundum Deposits". Gem Corundum. Research Gate: Mineralogical Association of Canada. pp. 37–38. ISBN .
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The Meanings of Corundum
Corundum is a gemstone with meaning and properties of seeing through the truth. It can grab beneficial information and lead you to the answer. It is good for people who have to deal with others often. This gemstone will be useful in service or sales. It is also useful when you want to know the true feeling of the person you love.
Corundum has meaning and properties of getting back pure mind. It is an effective gemstone when you want to get rid of distracting thoughts. It will be useful to refresh your mind. Corundum is a gemstone that can prevent you from lying to yourself. It is good to use when you feel like you are enduring something or making excuses a lot. You would be able to open up the right path by being honest to yourself.
Corundum is known for its power to increase its owner's knowledge. It is a famous gemstone to stimulate a desire for knowledge. All the knowledge and information you got will be beneficial to improve your skills. Corundum has meaning and properties to help achieving dreams easier. Studying and having more knowledge is necessary to accomplish goals. It is a powerful gemstone that can let you reach an ideal future.
Corundum is a kind of gemstone that changes its names according to its color. Blue Corundum is called Sapphire. Red Corundum is Ruby. Other than those, there are Yellow Sapphire and Green Sapphire. The meanings and the properties will change by its color, so please choose Corundum that is necessary for you.
Corundum is handled as jewelry. It will be more expensive than regular gemstones, so please have bigger budgets for it. The ones with beautiful color and high transparency are the pricy ones. The more beautiful ones are the more popular ones and the price will rise. It is usually made into necklaces, earrings, or bracelets to wear.
Corundum has meaning and properties of making the purpose of your life clear. It is a gemstone to correctly understand what your soul really wants. You would be able to have strong will to go towards your goal. Corundum can be used to make your soul mature. As you get closer to your purpose of life, your soul will grow, too.
Corundum is a gemstone that can make your life rich. From ancient times, it has been considered as a gemstone to feel the joy of life. It would attract success and prosperity by using it. You can have more relaxed mind from richness. It is a good gemstone to use when you want to adjust your emotion balance.
Chakra Chart of Corundum
Author Hayasaka Yoshino
Hayasaka Yoshino is a stone healer that usually works in Tokyo area, Japan. Yoshino is also famous as a fortune teller and seen over 10,000 people. "Fortune-telling counseling", which finds out the best gemstone for the client, received a high reputation. Representative director of LWE Co., Ltd.
Other Gemstone Recommendation
Gem Notes: Gemstone Information
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**Please note that all metaphysical or healing properties listed are collected from various sources. This information is offered as a service and not meant to treat medical conditions. Fire Mountain Gems and Beads® does not guarantee the validity of any of these statements.
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Ruby meaning corundum
Variety of corundum, mineral, gemstone
This article is about the mineral or gem. For other uses, see Ruby (disambiguation).
A ruby is a pink to blood-red colored gemstone, a variety of the mineralcorundum (aluminium oxide). Other varieties of gem-quality corundum are called sapphires. Ruby is one of the traditional cardinal gems, together with amethyst, sapphire, emerald, and diamond. The word ruby comes from ruber, Latin for red. The color of a ruby is due to the element chromium.
Some gemstones that are popularly or historically called rubies, such as the Black Prince's Ruby in the British Imperial State Crown, are actually spinels. These were once known as "Balas rubies".
The quality of a ruby is determined by its color, cut, and clarity, which, along with carat weight, affect its value. The brightest and most valuable shade of red called blood-red or pigeon blood, commands a large premium over other rubies of similar quality. After color follows clarity: similar to diamonds, a clear stone will command a premium, but a ruby without any needle-like rutileinclusions may indicate that the stone has been treated. Ruby is the traditional birthstone for July and is usually pinker than garnet, although some rhodolite garnets have a similar pinkish hue to most rubies. The world's most valuable ruby to be sold at auction is the Sunrise Ruby.
Rubies have a hardness of 9.0 on the Mohs scale of mineral hardness. Among the natural gems only moissanite and diamond are harder, with diamond having a Mohs hardness of 10.0 and moissanite falling somewhere in between corundum (ruby) and diamond in hardness. Sapphire, ruby, and pure corundum are α-alumina, the most stable form of Al2O3, in which 3 electrons leave each aluminium ion to join the regular octahedral group of six nearby O2− ions; in pure corundum this leaves all of the aluminium ions with a very stable configuration of no unpaired electrons or unfilled energy levels, and the crystal is perfectly colorless.
When a chromium atom replaces an occasional aluminium atom, it too loses 3 electrons to become a chromium3+ ion to maintain the charge balance of the Al2O3 crystal. However, the Cr3+ ions are larger and have electron orbitals in different directions than aluminium. The octahedral arrangement of the O2− ions is distorted, and the energy levels of the different orbitals of those Cr3+ ions are slightly altered because of the directions to the O2− ions. Those energy differences correspond to absorption in the ultraviolet, violet, and yellow-green regions of the spectrum.
If one percent of the aluminium ions are replaced by chromium in ruby, the yellow-green absorption results in a red color for the gem. Additionally, absorption at any of the above wavelengths stimulates fluorescent emission of 694-nanometer-wavelength red light, which adds to its red color and perceived luster. The chromium concentration in artificial rubies can be adjusted (in the crystal growth process) to be ten to twenty times less than in the natural gemstones. Theodore Maiman says that "because of the low chromium level in these crystals they display a lighter red color than gemstone ruby and are referred to as pink ruby."
After absorbing short-wavelength light, there is a short interval of time when the crystal lattice of ruby is in an excited state before fluorescence occurs. If 694-nanometer photons pass through the crystal during that time, they can stimulate more fluorescent photons to be emitted in-phase with them, thus strengthening the intensity of that red light. By arranging mirrors or other means to pass emitted light repeatedly through the crystal, a ruby laser in this way produces a very high intensity of coherent red light.
All natural rubies have imperfections in them, including color impurities and inclusions of rutile needles known as "silk". Gemologists use these needle inclusions found in natural rubies to distinguish them from synthetics, simulants, or substitutes. Usually, the rough stone is heated before cutting. These days, almost all rubies are treated in some form, with heat treatment being the most common practice. Untreated rubies of high quality command a large premium.
Some rubies show a three-point or six-point asterism or "star". These rubies are cut into cabochons to display the effect properly. Asterisms are best visible with a single-light source and move across the stone as the light moves or the stone is rotated. Such effects occur when light is reflected off the "silk" (the structurally oriented rutile needle inclusions) in a certain way. This is one example where inclusions increase the value of a gemstone. Furthermore, rubies can show color changes—though this occurs very rarely—as well as chatoyancy or the "cat's eye" effect.
Versus pink sapphire
Generally, gemstone-quality corundum in all shades of red, including pink, are called rubies. However, 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. Drawing a distinction between rubies and pink sapphires is relatively new, having arisen sometime in the 20th century. Often, the distinction between ruby and pink sapphire is not clear and can be debated. As a result of the difficulty and subjectiveness of such distinctions, trade organizations such as the International Colored Gemstone Association (ICGA) have adopted the broader definition for ruby which encompasses its lighter shades, including pink.
Occurrence and mining
Historically, rubies have been mined in Thailand, in the Pailin and Samlout District of Cambodia, as well as in Afghanistan, Australia, Brazil, Colombia, India, Namibia, Japan, and Scotland; after the Second World War ruby deposits were found in Madagascar, Mozambique, Nepal, Pakistan, Tajikistan, Tanzania, and Vietnam.
The Republic of North Macedonia is the only country in mainland Europe to have naturally occurring rubies. They can mainly be found around the city of Prilep. Macedonian rubies have a unique raspberry color. The ruby is also included on the Macedonian coat of arms. A few rubies have been found in the U.S. states of Montana, North Carolina, South Carolina and Wyoming.
Spinel, another red gemstone, is sometimes found along with rubies in the same gem gravel or marble. Red spinels may be mistaken for rubies by those lacking experience with gems. However, the finest red spinels can have values approaching that of an average ruby.
The Mogok Valley in Upper Myanmar (Burma) was for centuries the world's main source for rubies. That region has produced some exceptional rubies, however in recent years few good rubies have been found. In central Myanmar, the area of Mong Hsu began producing rubies during the 1990s and rapidly became the world's main ruby mining area. The most recently found ruby deposit in Myanmar is in Namya (Namyazeik) located in the northern state of Kachin.
In Pakistani Kashmir there are vast proven reserves of millions of rubies, worth up to half a billion dollars. However, as of 2017 there was only one mine (at Chitta Katha) due to lack of investment. In Afghanistan, rubies are mined at Jegdalek. In 2017 the Aappaluttoq mine in Greenland began running. The rubies in Greenland are said to be among the oldest in the world at approximately 3 billion years old. The Aappaluttoq mine in Greenland is located 160 kilometers south of Nuuk, the capital of Greenland. The rubies are traceable from mine to market.
Factors affecting value
Rubies, as with other gemstones, are graded using criteria known as the four Cs, namely color, cut, clarity and carat weight. Rubies are also evaluated on the basis of their geographic origin.
Color: In the evaluation of colored gemstones, color is the most important factor. Color divides into three components: hue, saturation and tone. Hue refers to color as we normally use the term. Transparent gemstones occur in the pure spectral hues of red, orange, yellow, green, blue, violet. In nature, there are rarely pure hues, so when speaking of the hue of a gemstone, we speak of primary and secondary and sometimes tertiary hues. Ruby is defined to be red. All other hues of the gem species corundum are called sapphire. Ruby may exhibit a range of secondary hues, including orange, purple, violet, and pink.
A naturally occurring ruby crystal
Natural ruby with inclusions
Clarity: Because rubies host many inclusions, their clarity is evaluated by the inclusions’ size, number, location, and visibility. Rubies with the highest clarity grades are known as “eye-clean,” because their inclusions are the least visible to the naked human eye. Rubies may also have thin, intersecting inclusions called silk. Silk can scatter light, brightening the gem’s appearance, and the presence of silk can also show whether a ruby has been previously heat treated, since intense heat will degrade a ruby’s silk.
Treatments and enhancements
Improving the quality of gemstones by treating them is common practice. Some treatments are used in almost all cases and are therefore considered acceptable. During the late 1990s, a large supply of low-cost materials caused a sudden surge in supply of heat-treated rubies, leading to a downward pressure on ruby prices.
Improvements used include color alteration, improving transparency by dissolving rutile inclusions, healing of fractures (cracks) or even completely filling them.
The most common treatment is the application of heat. Most rubies at the lower end of the market are heat treated to improve color, remove purple tinge, blue patches, and silk. These heat treatments typically occur around temperatures of 1800 °C (3300 °F). Some rubies undergo a process of low tube heat, when the stone is heated over charcoal of a temperature of about 1300 °C (2400 °F) for 20 to 30 minutes. The silk is partially broken, and the color is improved.
Another treatment, which has become more frequent in recent years, is lead glass filling. Filling the fractures inside the ruby with lead glass (or a similar material) dramatically improves the transparency of the stone, making previously unsuitable rubies fit for applications in jewelry. The process is done in four steps:
- The rough stones are pre-polished to eradicate all surface impurities that may affect the process
- The rough is cleaned with hydrogen fluoride
- The first heating process during which no fillers are added. The heating process eradicates impurities inside the fractures. Although this can be done at temperatures up to 1400 °C (2500 °F) it most likely occurs at a temperature of around 900 °C (1600 °F) since the rutile silk is still intact.
- The second heating process in an electrical oven with different chemical additives. Different solutions and mixes have shown to be successful, however mostly lead-containing glass-powder is used at present. The ruby is dipped into oils, then covered with powder, embedded on a tile and placed in the oven where it is heated at around 900 °C (1600 °F) for one hour in an oxidizing atmosphere. The orange colored powder transforms upon heating into a transparent to yellow-colored paste, which fills all fractures. After cooling the color of the paste is fully transparent and dramatically improves the overall transparency of the ruby.
If a color needs to be added, the glass powder can be "enhanced" with copper or other metal oxides as well as elements such as sodium, calcium, potassium etc.
The second heating process can be repeated three to four times, even applying different mixtures. When jewelry containing rubies is heated (for repairs) it should not be coated with boracic acid or any other substance, as this can etch the surface; it does not have to be "protected" like a diamond.
The treatment can be identified by noting bubbles in cavities and fractures using a 10× loupe.
Synthesis and imitation
Artificial ruby under a normal light (top) and under a green laser light (bottom). Red light is emitted
In 1837, Gaudin made the first synthetic rubies by fusing potash alum at a high temperature with a little chromium as a pigment. In 1847, Ebelmen made white sapphire by fusing alumina in boric acid. In 1877, Edmond Frémy and industrial glass-maker Charles Feil made crystal corundum from which small stones could be cut. In 1887, Fremy and Auguste Verneuil manufactured artificial ruby by fusing BaF2 and Al2O3 with a little chromium at red heat. In 1903, Verneuil announced he could produce synthetic rubies on a commercial scale using this flame fusion process, later also known as the Verneuil process. By 1910, Verneuil's laboratory had expanded into a 30 furnace production facility, with annual gemstone production having reached 1,000 kilograms (2,000 lb) in 1907.
Other processes in which synthetic rubies can be produced are through Czochralski's pulling process, flux process, and the hydrothermal process. Most synthetic rubies originate from flame fusion, due to the low costs involved. Synthetic rubies may have no imperfections visible to the naked eye but magnification may reveal curved striae and gas bubbles. The fewer the number and the less obvious the imperfections, the more valuable the ruby is; unless there are no imperfections (i.e., a perfect ruby), in which case it will be suspected of being artificial. Dopants are added to some manufactured rubies so they can be identified as synthetic, but most need gemological testing to determine their origin.
Synthetic rubies have technological uses as well as gemological ones. Rods of synthetic ruby are used to make ruby lasers and masers. The first working laser was made by Theodore H. Maiman in 1960. Maiman used a solid-state light-pumped synthetic ruby to produce red laser light at a wavelength of 694 nanometers (nm). Ruby lasers are still in use. Rubies are also used in applications where high hardness is required such as at wear-exposed locations in modern mechanical clockworks, or as scanning probe tips in a coordinate measuring machine.
Imitation rubies are also marketed. Red spinels, red garnets, and colored glass have been falsely claimed to be rubies. Imitations go back to Roman times and already in the 17th century techniques were developed to color foil red—by burning scarlet wool in the bottom part of the furnace—which was then placed under the imitation stone. Trade terms such as balas ruby for red spinel and rubellite for red tourmaline can mislead unsuspecting buyers. Such terms are therefore discouraged from use by many gemological associations such as the Laboratory Manual Harmonisation Committee (LMHC).
Records and famous examples
- The Smithsonian's National Museum of Natural History in Washington, D.C. has some of the world's largest and finest ruby gemstones. The 23.1-carat (4.62 g) Burmese ruby, set in a platinum ring with diamonds, was donated by businessman and philanthropist Peter Buck in memory of his late wife Carmen Lúcia. This gemstone displays a richly saturated red color combined with an exceptional transparency. The finely proportioned cut provides vivid red reflections. The stone was mined from the Mogok region of Burma (now Myanmar) in the 1930s.
- In 2007, the London jeweler Garrard & Co featured a heart-shaped 40.63-carat ruby on their website.
- On December 13/14, 2011, Elizabeth Taylor's complete jewelry collection was auctioned by Christie's. Several ruby-set pieces were included in the sale, notably a ring set with an 8.24 ct gem that broke the 'price-per-carat' record for rubies (US$512,925 per carat – i.e., over US$4.2 million in total), and a necklace that sold for over US$3.7 million.
- The Liberty Bell Ruby is the largest mined ruby in the world. It was stolen in a heist in 2011.
- The Sunrise Ruby is the world's most expensive ruby, most expensive colored gemstone, and most expensive gemstone other than a diamond. In May 2015, it sold at auction in Switzerland to an anonymous buyer for US$30 million.
- A synthetic ruby crystal became the gain medium in the world's first optical laser, conceived, designed and constructed by Theodore H. "Ted" Maiman, on 16 May 1961 at Hughes Research Laboratories. The concept of electromagnetic radiation amplification through the mechanism of stimulated emission had already been successfully demonstrated in the laboratory by way of the maser, using other materials such as ammonia and, later, ruby, but the ruby laser was the first device to work at optical (694.3 nm) wavelengths. Maiman's prototype laser is still in working order.
Historical and cultural references
- The Old Testament of the Bible mentions ruby many times (along with a catalogue of other precious stones) in the Book of Exodus, and many times in the Book of Proverbs, as well as various other times.
- An early recorded transport and trading of rubies arises in the literature on the North Silk Road of China, wherein about 200 BC rubies were carried along this ancient trackway moving westward from China.
- Rubies have always been held in high esteem in Asian countries. They were used to ornament armor, scabbards, and harnesses of noblemen in India and China. Rubies were laid beneath the foundation of buildings to secure good fortune to the structure.
- A traditional Hindu astrological belief holds rubies as the "gemstone of the Sun and also the heavenly deity Surya, the leader of the nine heavenly bodies (Navagraha)." The belief is that worshiping and wearing rubies causes the Sun to be favorable to the wearer.
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