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.

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What Colours can Corundum Have, How are These Colours Generated, and What Gem Varieties Result?

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.

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What Does it Look Like Rough?

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.

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What Does Corundum Look Like When Cut?

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.

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How is it Recognized and Distinguished from Other Materials?

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.

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How is Corundum Valued?

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.

Rubies

Size, carats	Approximate value per carat, USD
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

 

Sapphires

Size, carats	Approximate value per carat, USD
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:

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.

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What are Common Treatments for Corundum?

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 solid inclusions 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.

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Can Corundum be Produced Synthetically?

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.

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What are its Imitations?

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.