Wednesday, 6 February 2013

Introduction to Corundum Aluminum Oxide

Inclusions in the shape of a butter-fly's wing in a blue sapphire (photomicrograph). Aluminum oxide: The name is probably derived horn an old Indian word, corund, which referred loan unknown mineral or gem.

Crystal system: Trigonal.

Appearance: It occurs in serniopaque masses similar to whitish or grayish vein quartz, but also in distinct, prismatic or tapered crystals, with close transverse striations some of which resemble elongated bipyramids. Often 0paque or translucent corundum can be partially or perfectly trans-parent All the colors of the spectrum are possible, from red to yellow, green, blue, and violet in addition, the stones may be Pink, gray, black, or colorless and all the shades between Brightly colored, transparent, translucent, or Semiopque varieties make tubby aesthetic and valuable gems. Because of its hardness and resistance to chemical attack, corundum is often found at alluvial de' posits on the form of pebbles that retain clear indications cot their original crystal shapes.

Physical property:  Corundum has a hardness of 9, the highest in the mineral world after diamond. The density 0 approximately 4.0g/crn3. The refractive indices are about n€ 1.760. n  1.769 Parting parallel to the basal plane is sometimes visible, with an appearance 0f cleavage

Genesis: It is formed by contact metamorphism between alurnina-rich magmas (and related pegmatite’s) and lime-stone, or by regional metamorphism of alumina-rich, Silica-peer rocks.

Blue, yellow pink etc. sapphires. Occurrence: The least attractive variety or corundum, known as emery (usually a corundum-magnetite mixture) and used as and abrasive, is mainly found in Greece, the United States, and Australia The gem variety come chiefly horn So Lanka, Thailand, Cambodia, BUM, and Austral., with smaller deposes in India, Tanzania, and the United Stales


The roost valuable variety of corundum Is ruby The nurse comes from the Latin rubrun, "red.”  Like other red stones, it has also been called carbunculus, or carbuncle, meaning a small coal or ember.

Appearance: The color varies from 10ry vermilion to violet red, but because rubes are pleochroic, different colors are also found in the same stone, bright or sometimes brick ted in one direction, tending to carmine in the other. The colour is also accompanied by marked fluorescence, which is stimulated by ordinary, artificial light and above all. by the ultraviolet rays of direct sunlight Thus rubies turn brighter red under such light and the purplish ones look -redder '' It the color is too pale, they are no longer called rubes, but tank sapphires, if it Is mete violet than red, they are known as valet sapphires. 

But it is hard to establish precise limes, as all Me Intermediate shades are possible The brightest red and thus the most valuable rubies (usually from Burma) Orion have areas hill of Inclusions in the form of minute rutile needles (or straws), which Interfere with the light, producing a distinctive silky sheen known, in fact, as silk. 

 Ruby When the silk is not heavy, the stones are Clearer, more attractive, and even more valuable. Other, mainly crystalline inclusions are normally found as well. Rubles at this type are not usually more than a few carats in weight. The rare exceptions generally coo fain copious inclusions. Violet red, sometimes quite dark, rubies come principally from Thailand. The lyric most often round on the market nowadays, they can be several Carats In weight. They are normally clearer, without patches of silk. While good-sized clear stones are found, specimens with many inclusions are commonly said as well Rubes are, weirdly (peen a mined cut, which is generally oval. hut can be round or, more rarely. other shapes. In the past they were given a cabochon cut, like all stones outstanding for their color. Today, however, this cut is reserved tor lees transparent stones with numerous inclusions. 

Distinctive features: Rubles can often be distinguished by their immediately visible characteristics: a fairly obvious pleochroism, a distinct brightening or color in strong light. the silk effect (where present), and a considerable luster. While spinet can be a similar color and has a similar luster, it is not pleochroic, turns much less bright in strong light mini never displays the silk effect. Red garnet' is not Pleochroic and the motor does not brighten in strong light 11 has a similar luster, but when given a faceted cut often displays dark, blackish areas within the stone. 

Jewelry with ruby and diamonds. Red tourmaline is usually a completely different shade, but can he very similar, with a pleochroism comparable to that of ruby. It does not however, brighten in strong fight and this can be sufficient Indication to warrant testing. It`s physical properties, which are quite different. The other red gemstone mentioned also flitter physically from ruby. Some caution is needed with gainers, which show wide variations in both density, which in some cases coincides with that of corundum, and refractive index, which can coincide with one of the figures for corundum. Garnet, however, is singly refractive, and examples with an index in the region of r TO have a lower density than that of ruby.

Occurrence: The rubies with the finest color come from the Mogok region in Burma. These am most truly vermilion, though they still have a touch of carmine. Thailand. however, is today the main source of rubies Thai rubies ore usually slightly less attractive, a tat darker with a violet tinge, but they Mimi have fewer inclusions. Rubies are also found in Sri Lanka, but in only small quantities. Often pale, almost pink, they can be attractive, with an appearance that is both brilliant and lively. Small quantities of very fine rubies also come nom the area of Cambodia on the border with Thailand, while rather opaque specimens, mainly al n. tenor quality, are found in India and Pakistan. Tanzania and neighboring countries have also been mining rubies la a few years. Some of the rubies found. In these countries are almost as finely colored as those from Burma, with inclusions similar to rubies from Thailand, while others are semiopaque and of very limited value.

Value: The highest quality, best colored and most transparent stones (usually from Burma). Weighing, for example 3 to 5 carats, can be as valuable as diamonds, or river more so. Very good quality rubles of even greater weight are extremely rare and fetch exceptionally high prices. Good quality stones of at least 2 carats (a bit more violet 8n color and usually from Harland) are still quite valuable (Particularly the more transparent ones). The price falls considerably for stones of less than a carat, which are too dark in color, and nave inclusions clearly visible to the naked eyes.

The silk effect in a ruby caused by minute rutile needle inclusion.Simulates and synthetics: Ruby has very occasionally been imitated by glass, which has a rather different. Freely color and an inferior 'aster, It has sometimes been imitated by doublets, with the top part consisting of garner. In provide luster, hardness, and natural looking inclusions and the bottom part of red glass, fused rather than cemented to the garnet layer. But such imitations are uncommon. Synthetic ruby has been produced from the beginning of the twentieth century and was the first synthetic Gemstone to be manufactured on an industrial scale To make these synthetic stones harder to distinguish from some natural rubies with numerous inclusions, thee have sometimes been fractured internally by healing and rapid cooling. More recently, doublets imitating rubies have also been produced in the Orient. 

The top pail of these doubters consists of poorly colored (usually pale preen of yellow) natural corundum with obvious, typical inclusions, and the lower part is synthetic ruby, hold to rho corundum by trans. parent cement. The effect is highly deceptive: the 108.ftf, leg presence of natural inclusions and characteristic Mate, Combined with a color which is not perfect, but is normal for the majority of rubies.


This is the blue variety of corundurn, the camels probably arrived, though the Latin sapphires and Greek Mullein's, from a Sanskrit word. As with other gem names, however, the Latin sapphires did not originally denote the now it is associated with today. Judging by the description of Pliny the Elder, it almost certainty referred to what is note known as lapis lazuli, rather than corundum.

"Rosser Reeves" star ruby, 138 Cl. Appearance: Sapphires can be a very dark blue, to the point of seeming dense and blackish from a distance, sometimes accompanied by a blue to dull green pleoochrroism, which is may visible horn the side in cut stones. They may also be a strong hut not too bright blue, easily recognizable from a distance, this beam lie Ideal color. Other possibilities are light, usually bilphl, blue, with the color unevenly distributed: palish blue or, finally, bee Wily aciolot tinge, at least in bright light. Like all corundum. seetahre always has good luster.

Some sapphires display clearly defined streaks of paler color, in contrast In a dark ground. Others have areas with o slightly silky sheen, which are not °lowly a:elevated. Still other, uncommon varieties assume a distinct, milky appearance in strong light, with a marked Increase In colorintensity. Inclusions are, as a rule, less obvious in very dark stones, due to their general lack of transparency, whereas medium to large, pale stones often show distinct "veils- or "feathers" caused by very fine inclusions and foreign crystals, which are sometimes transparent, sometimes dark, submetallic, and opaque, and, very occasionally bright red. 

Sapphires in various shades of blue. Sapphires are usually given oval or less frequently, round, mixed cuts, but rectangular or square, stet cuts, with or without trimmed corners, are also possible The cabochon cut is used as well, although less frequently than in the past. Nowadays it is generally reserved for stones full of inclusions or those in which the color is concentrated in a few streaks on a light ground. In the lane' case, in fact, the cabochon cut gives the color a more uniform appearance. Stones weighing several carats or eve 10 to 20 carats in the case of light-colored specimens are not uncommon. 

Distinctive features

Like other types of corundum, sapphires have a striking luster. The color is also quite distinctive, whether or not clear blue-green pleochroism is visible. The overall appearance is very important. For example, a deep blue color with distinct blue-green pleochroism one internal streaks straight across or at an angle of 120°, combined with the powerful luster of corundum, indicates a sapphire of Australian origin. A slightly patchy, blue color with imperceptible pleochroism and strong transparency showing veillike inclusions and a slight silk effect, still with excellent luster, denotes a sapphire from Sri Lanka. Corn flower to deep blue in a stone without obvious inclusions but of slightly milky appearance, acquiring a distinct fullness of color in bright light, is characteristic of the rare sapphires from Kashmir. 

Sapphire from Sri Lanka.
Of the other blue stones, tanzanite always shows a hint of violet, fairly obvious pleochroism, and less luster than sapphire. Cordierite apart from being less lustrous and violet or gray blue has striking pleochroism from blue to an unmistakable dratted yellow. Strongly colored specimens of indicolite tourmaline are often an attractive greenish blue, with a pleochroism ranging from blue to green, but the green is very different from that of sapphire which, when it is present, is always dull or yellowish. 

Still on the subject of pleochroism tourmaline, the direction corresponding to the blue colon shows a characteristic lack of transparency. While blue zircon has a luster similar to that of sapphire, it is an electronic blue or blue-green unlike that of any other gemstone Furthermore, its strong birefringence, seen in a clear duplication of the facet edges when viewed through the stone with a lens, would remove all trace of doubt; sapphire is doubly refractive as well, but to a much lesser degree. In the rare cases when blue spinet is not cloudy blue or violet gray, but a vivid mid-blue, it can look very much like sapphire, partly on account of its strong luster. In this case, it can only be distinguished by its physical characteristics; establishment of single refractivity or measurement of the density or refractive index should suffice.


Rough corundum of various colors.The best sapphires were discovered in a small deposit in Kashmir in 1880, in a remote mountain area which has now probably been exhausted. Very fine sapphires are also found in Burma, but in limited quantities.

Appreciable quantities of light- and bright-blue sapphire are found in alluvial deposits on the island of Sri Lanka. These are always attractively (if sometimes patchily) colored, the richest versions being very similar to the Burmese sapphires and equally valuable. The sapphires of Sri Lanka are also famous for the variety of inclusions they display: long, thin rutile needles, like very fine silk; soft, liquid inclusions arranged in the form of veils, lace, and feathers; striking inclusions with a moving bubble, like a spirit level; zircon crystals with small stress cracks radiating from them, and various other types of transparent crystals.

Sapphires are also mined in Thailand and neighboring Cambodia. These are generally pleasing to the eye, though often rather deeply colored. But most sapphires come from Australia, which has numerous deposits of deeply colored stones, sometimes too dark, in most cases with blue-green pleochroism. These are the least valuable, but most widely available on the market. Less important sources are the United States (Montana), Tanzania, and Malawi.


Pink sapphire 0.54 ct. Sri Lanka.
The finest stones, weighing at least several carats, are almost as valuable as diamonds and rubies and are hence very highly priced. This is particularly true of most sapphires from Kashmir, many from Burma, and some from Sri Lanka, Cambodia, and Thailand. But when the color is too dark, blackish or greenish blue or a bit too pale, the value falls sharply, to that normal for secondary gems. Inclusions obvious to the naked eye also lower the price. Small stones (of a fraction of a carat) are modestly priced and readily available. Large ones (from more than ten to several tens of carats), although not common, are much less rare than rubies of this size.

Simulants and synthetics

Sapphire has been imitated by dark to cobalt blue glass, but particularly by doublets with a top part consisting of red almandine garnet, which is very hard and lustrous, with natural inclusions, and a bottom part of dark-to-cobalt blue glass, welded together, not glued. It has also been imitated in the past by synthetic blue spinet, which is brightly colored but emits strange red gleams in bright light. Synthetic sapphire has likewise been produced for many years now, mainly by the Verneuil flame fusion method. Recently, doublets have been produced consisting of a top portion of light green or yellow-green natural corundum with visible inclusions and a lower portion of synthetic sapphire, held together by transparent cement. The visible inclusions and typical corundum of the top part, along with the color, make these doublets very convincing at first sight.

Liquid and solid inclusions in a violet sapphire (photomicrograph). Since the end of the 1970s, greater knowledge of the nature and causes of color in gemstones has enabled the modification of this feature by various procedures. The most recent and important techniques, intact, relate to the blue coloration of sapphire. One method is to subject very pale blue, almost colorless stones with numerous silklike rutile inclusions to prolonged heating at temperatures in the region of 1500-1600°C In a reducing environment. This "reactivates" the titanium in the rutile, which reacts with the traces of iron in the sapphire. In this way, the silk is absorbed, while the trivalent titanium and iron thus formed, which are responsible for the blue coloration of sapphire, greatly intensify the color of the stone. 

This treatment is now very widespread and more or less reproduces the sequence of events that occurred when many sapphire crystals were formed. As a result, it is not always possible to distinguish a completely natural sapphire from one whose color has been intensified in this way, and they are treated as one on the market. According to another procedure, however, colorless, pale yellow or pale green stones are covered in a paste consisting of iron and mainly titanium compounds. The specimens are then heated to a temperature of about 1700°C for perhaps several days. 

The iron and titanium oxides slowly infiltrate the stones to a depth of about one millimeter, producing a deep blue coloration. The stone then has to be repolished (the surface having been damaged by heating to near melting point). Hence part of the colored layer is removed, leaving a very small thickness. This procedure is surprisingly common and involves the introduction of additives as colorants. It is universally regarded as fraudulent if the treated stones are then offered for sale as natural stones, as is often the case.

Pink sapphire

This is the name given to the pink variety of corundum, red corundum generally being known in English-speaking countries as ruby, blue corundum as sapphire, and any other shades as sapphire combined with the appropriate color pink sapphire, yellow sapphire, green sapphire, etc. Pink sapphire and ruby are regarded as two different varieties, despite the fact that the only difference is their depth of color. The same does not apply to tourmaline, both the pink and red forms of which are known as rubel-lite, or to sapphire and emerald, which keep their names even for paler specimens.


Pink sapphire may range from a very deli-cafe pleasing, lively pink, without any overtones, to pink with a slight violet tinge; but all gradations of color are possible, from those tending toward ruby to those tending to) ard violet sapphire. Like all forms of corundum, it has very good luster. It is normally given a mixed, oval cut and sometimes has fine inclusions and liquid veils in lacelike formations, characteristic of the corundum of Sri Lanka. from where most pink sapphire comes. Stones of several carats are normal, but specimens of 10 carats or more are rare.

Distinctive features

Pink sapphire's most striking characteristic is its luster, common to all corundum and most noticeable in light-colored specimens. It is usually, though not always, a livelier, more attractive color than tourmaline; and as is often the case with paler stones, the physical characteristics generally have to be measured to tell them apart. In the case of pink stones, measurement of the density by Means of heavy liquids may be sufficient.


Pink sapphire comes almost exclusively hem Sri Lanka; much more rarely from Burma.


Although the "minor" varieties of corundum are always a lot less valuable than rubies and sapphires, pink sapphire is more highly prized than the yellow, green, and violet varieties. as it is so attractive. It is one of the most valuable secondary gems.

Simulants and synthetics

Ring with yellow sapphire and small emeralds. Pink sapphire has not really been imitated, but it has been produced synthetically by the Verneuil flame fusion method. The synthetic form, like that of yellow sapphire, is extremely well disguised and it is very hard to distinguish it from the natural varieties.

Violet sapphire

This is the name now given to the violet variety of corundum. It was formerly known as "oriental amethyst," on account of its color: but this name has now been abandoned in favor of the debatable, but mineralogically more precise, term used for all minor forms of corundum.


The typical color is a definite violet, like amethyst which is very attractive and tends to turn reddish violet in the sun or bright light. But all gradations of color are possible from the violet blue of some sapphires to the violet red of some rubies and the violet pink of some pink sapphires. Whatever the exact shade, it is always a very pleasing color, and has perhaps been less appreciated than it deserves because of the association of violet with mourning and sorrow in the West. As always with corundum, it has very good luster, most evident in lighter gem-stones. It is usually given a mixed, oval cut. Stones weighing a few carats are often found, but those of 10 carats and rnore are rare.

Distinctive features

Green sapphire, 1.40 ct. When the color of the sapphire tends to be blue violet. Red-violet or pink-violet (the first turning more violet, the others, redder, in sunshine or bright light), it is fairly distinctive, but not so when it is a true violet or violet with a slight hint of red. Some very fine amethysts look very similar to such sapphires, even to the reddish tinge. The characteristic luster of corundum is another distinctive feature, but here too, care is needed, as amethyst can bear a close resemblance. Violet sapphires are easily distinguished, however, by their physical characteristics. If the density is measured with a heavy liquid such as methylene iodide, corundum rapidly sinks, while amethystine quartz floats.


Many violet sapphires, particularly the paler pinker ones, come from Sri Lanka. Much smaller quantities are also found in Thailand and Burma.


Despite its considerable aesthetic qualities, violet sapphire. Is not widely appreciated. It is accorded much the same value as other, secondary gemstones, being some-what more valuable than yellow sapphire (which is more plentiful).

Simulants and synthetics

Ring with green sapphire, 1.83
Violet sapphire does not appear to have been imitated: but from the time synthetic corundum was first produced; various shades of violet have been manufactured, along with the other varieties. Large quantities of violet synthetic corundum are still produced, faceted into every imaginable shape, from oval or round mixed cuts to true brilliants and square and rectangular step cuts. The curious thing is that these stones, which are often of considerable size and weight (even 10 to 20 carats), are sold throughout the world under the name of alexandrite. An extremely rare torte of green chrysoberyl that changes color to red, and is therefore not even vaguely like synthetic corundum.

Green sapphire

This is the name given to the relatively uncommon, green variety of corundum. In the nineteenth century it was also known as "oriental emerald," just as violet and yellow corundum were called "oriental amethyst" and "oriental topaz." These names stemmed from a scant knowledge of mineralogy among gem merchants and have now been abandoned.


Due to its iron content, green sapphire is generally quite a strong, bright green color, sometimes with green to bluish green or yellowish green pleochroism. Individual stones are sometimes cut at different angles Irons the rough crystal, to bring out the best color. The luster is very good, as with all corundum. The mixed, oval cut is the most common, but square or rectangular step cuts are also used. These gems are usually small to medium-sized and rarely exceed a few carats.

Distinctive features

Colorless sapphire, 1.50 ct.
The color is quite distinctive, especially combined with the particular luster of corundum. While green tourmaline can, at first glance, look fairly similar in color, it has more pronounced pleochroism, the direction corresponding to the bluer green often seeming rather opaque, and it is always, or nearly always, given a rectangular, step cut strictly aligned to the elongated shape of the prism. Green zircon can be quite similar to green sapphire in both color and luster, but it has far less obvious pleochroism and sometimes much stronger birefringence, easily detectable with a lens. Many green zircons, however, have weak or virtually nonexistent birefringence. In such cases, their density is also very similar to that of corundum, so the physical characteristics will need careful checking to establish a distinction.


Green sapphire comes mainly from Australia, but it is also found in the United States (Montana) and Thailand.


As for all forms of corundum except ruby and Sapphire, its value is quite low. It is perhaps worth a bit more than yellow sapphire, due to its greater rarity and the difficulty of finding stones of any size.

Simulants and synthetics

Photomicrograph of the star in a star sapphire. Not being widely known or appreciated, this stone is not often imitated. But green synthetic corundum has occasionally been produced: it is of a brighter color than the natural mineral. Bluish green synthetic spinet has also been produced, whether to imitate green sapphire or zircon it is hard to say, as it bears little resemblance to either.

Colorless sapphire 

When the crystal structure of corundum does not contain trace elements that act as colorants, it is completely clear although this form is the least known and appreciated on the gem market. The name of loucosapphire, coined fairly recently. is derived from the Greek loykos, meaning "white."


It is perfectly colorless or occasionally has a slight yellow tinge and has the typical luster of corundum. It can have fine veillike and lacelike liquid inclusions and even areas that look cloudy in bright light, due to the presence of fine, crossed needles or minute straws of r utile; it may also have small crystalline inclusions with minute cracks radiating from them, like much corundum from Sri Lanka. It is given a round for almost round) mixed cut, or a slightly modified brilliant cut, having mainly been used as a substitute for diamond.

Distinctive features

Liquid inclusions in a sapphire (photomicrograph). Despite its luster, it is very easily distinguished from diamond, which it was once meant to simulate. It has less dispersion and fire, plus weak birefringence, where visible. Diamond, of course, Is singly refractive. The taint yellow coloration of some colorless sapphires can, however, make them more plausible as imitations. Colorless corundum is distinguished from colorless zircon by the pronounced birefringence of the latter,


Colorless sapphire comes mainly from Sri Lanka, where Quantities of light-colored corundum are found, but is apparently also found in Burma and else-where. It is of little interest as a gemstone and is chiefly used tor industrial purposes as an abrasive.


Very low, but hard to quantify, as it now has scarcity value for collectors and amateurs.

Stimulants and synthetics

Jewelry with sapphire.Colorless corundum has not been imitated, but was formerly used to imitate diamond. It has been produced synthetically and as with the other pale varieties, the internal features characteristics of Verneuil synthetic corundum are very well disguised. Synthetic leucosapphire is used mainly as a diamond simulant, particularly for small stones, in which the differences’ are less apparent.  

Star rubies and sapphires. Different-colored astenae

Corundum often contains very fine needles of futile (Ti02) arranged in intersecting lines in accordance with the symmetry of the crystal. These apparently develop when the stone is formed: as the temperature falls, the TiO2 is no longer soluble In the AI,03 and forms separate crystals. When the needles inside the corundum are particularly numerous, and the stone is cut en cabochon, with its base (or widest diameter) parallel to the base of the prism, a silk-type reflection is visible in bright light: it is fairly mobile and has the appearance of a six-rayed star, the closer and thicker the rattle needles inside, the more clearly this stands out. When reasonably pronounced, this effect is considered attractive, contributing in the past to the aura of mystery surrounding some gemstones.


The ''Star Asia" star sapphire, 330 ct. The most striking phenomenon of rubies and sapphires is the development of the six-ray star, arranged in perfect symmetry, which shifts its center as the stone is moved. It is clearly visible under a single light source Such as the sun or a lamp; much less spin diffuse light. If two or more powerful light sources are set close together, as many stars (their centers not far apart) can be seen in the stone. Each light produces its own star, which is basically a reflection. The effect is usually less pronounced in more transparent stones. The ground color can be ruby red (or an almost grayish, dull red), in which case it is known as a star ruby, or sapphire blue, in which case it is known as a star sapphire, but it can also be blue-gray smoke gray, or blackish, all of which come under the name of Astoria, a generic term also applied to ruby and sapphire. 

Such stones are invariably given a round or oval cabochon cut. The most highly prized are the ruby-colored (provided they are not the opaque, grayish red of some Indian rubies) and sapphire blue varieties. The others are less valuable, but still sought after, provided the star is clearly visible and they are not too small (3 or 4 carats, at least). Some star stones may weigh 10 carats or more.

Distinctive features

 A ruby in the form of a wafer-worn pebble from Sri Lanka
The star with its distinctive mobility is characteristic, having six rays, unlike star diopside, for example, which has four. But to be certain of distinguishing star corundum from the widely divergent (but very few) other gems which can display the phenomenon of asterism, one normally has to measure the physical proper-ties.


Rare but magnificent star rubies and sapphires are found in Burma, although most star corundum’s come from Sri Lanka, usually being light blue or gray. Dark asteriae are found in Australia, and dull red, opaque specimens are found in India. Despite being rubies and displaying the phenomenon of asterism, these stones are not very attractive.


Star rubies and sapphires of good or even above average color are distinctly valuable, as much so as faceted stones of similar color. The value of the grayish or dark asteriae is lower, though not much, for unusually fine specimens. On the other hand, comparatively small stones of insipid color or with a poorly defined star are worth a great deal less, and the same applies to dull-colored rubies, which often have a broad, smudgy star.

Simulants and synthetics

Blue-gray asteria, 1.50 ct. Sri Lanka.
Because of their undoubted attraction, star stones have been imitated in various ways. Efforts have been made to produce a star by engraving It on the base of a cabochon, or lining the base with a sheet of metal engraved with a six-rayed star. Milky quartz, which exhibits a weak form of the same type of asterism, has also been used the base of the cabochon being covered with blue lacquer to give color to the stone and increase the contrast with the star; but the effect is somewhat different from that of natural star stones. 

In the last few decades however some manufacturers of synthetic corundum (using the Verneuil method) have found a way of producing both red and blue star stones with very pronounced stars (more pronounced than the natural versions), which are not as a rule too transparent and have an attractive, lively color; and these have been a great success in the United States.

Writer – Curzio Cipriani & Alessandro Borelli
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