Chrysoberyl

Chrysoberyl




A most superb chrysoberyl, nearly of a circular, and of a deep yellowish green tint similar to peridot, and most admirably cut like a brilliant. This extra ordinarily fine gem is of the greatest transparency and brilliancy, and is free from any speck or flow; its uncommonly large size and great perfection entitle it to be called a matchless specimen. It is certainly to be considered one of the rarest specimens of chrysoberyl, there being very seldom a stone of this size and weight found in this class of gems that exhibits such perfection.

Chrysoberyl (pronounced KRIS-dh-barrel), which is derived from two Greek words meaning "golden" and "Beryl", is one of the most interesting, yet least known, of the important gems. For those who adhere to the questionable policy of referring to gemstones as "precious" and "semiprecious" chrysoberyl is difficult to classify, because, as with jade in the so-called precious class. Probably the best-known variety is the cat's-eye. With its silky luster and sharp "eye", it is the most beautiful chatoyant stone known in nature. In the Orient, cat's-eye is highly revered as a preserver of good fortune, the belief being that it guards the owner's health and protects him from poverty. At One time, it was sometimes carved in the shape of an animal's head, to emphasize the unusual appearance of chatoyancy. The Hope Collection contained an enormous cat's-eye that was carved to represent an altar surmounted by a torch; it was hemispherical in shape and measured about one and one half inches in diameter. The native of Ceylon consider cats-eye to be a potent charm against evil spirits. British royalty has long favored the gem for engagement rings. The American Museum of Natural History boasts a superb cat's-eye that weights 47.8 carats. A number of fine stones have appeared in sizes over 100 carats; one weighing more than 300 carats was sold several years ago.

Alexandrite, which alternates with pearl and moonstone as the birth-stone for the month of June, is particularly interesting because of its color change; the more poetic writers have referred to it as "an emerald by day and a ruby by night". This variety received its name from the fact that it was supposedly discovered on the birthday of Alexander II, Czar of Russia, in 1830. Because of this, and because red and green are the colors of the Russian Imperial Guard, alexandrite has been a particular favorite in that country. Since this gem very rarely occurs in large sizes, the 43 and 27.5 carat specimens of exceptional quality in the British Museum rate particular attention.

The transparent, nonphenomenal chrysoberyls are more commonly encountered in large sizes. The American Museum of Natural History has a 74.44 carat, yellow-green, emerald-cut stone of exceptional quality; it is believed to be the largest cut specimen of this desirable color in any collection.

Varieties of Chrysoberyl

Among the varieties of chrysoberyl are some of the most interesting of the colored stones; their popularity seems to be growing significantly. It was not long ago that cat's-eyes were rarely seen and alexandrite was unknown, except to the gem fancier. Some of the varieties that display no optical phenomena are still little known, but cat's-eye and alexandrite are terms that are at least vaguely familiar to many customers today.

In contrast to its position on the naming of most of varieties of the gem species studied to date, the Institute recommends the use of two variety names with chrysoberyl. Both alexandrite and cat's-eye are so clearly associated with specific phenomena that to fail to use these terms would be to lead to needlessly repeated descriptions; when the variety name by itself conveys the meaning clearly to gem-stone students and collectors.

Alexandrite

Chrysoberyl that exhibits a distinct change of color from red under candlelight or tungsten-filament incandescent bulbs to green in daylight or fluorescent illumination is called alexandrite. The name is applicable only when a distinct color change is visible. Many stones are offered as alexandrite, even though they do not have a distinct color change.

Briefly, a study of the absorption spectrum of alexandrite discloses that there is a broad band of absorption from about the orange through the yellow-green region of the spectrum, with almost no absorption of either red or green. Since daylight is strong in the shorter wave-lengths and candle-light and incandescent light are strong in the longer wavelengths, there are distinct differences in the color of the s tone under the two light sources.

The colors expected from a fine alexandrite have been likened to those of ruby and emerald, but even the finest specimens fail to approach the intensity of color of these two stones. The best artificial-light color is a rich, dark red to purplish red, with a minimum of brownishness; however, brownish red is usually present. The daylight color should be an intense, lightly yellowish or bluish green, but it also is of medium to low intensity with a slightly brownish cast. An olive green is not uncommon.

Cat's-Eye

The finest example of chatoyancy among gemstones is that displayed by chrysoberyl. The optical phenomenon beautifully reproduces the appearance of the narrowed pupil in the eye of a Cat. Chatoyancy may occur in any color of chrysoberyl, but the most highly prized is slightly greenish yellow or brownish yellow, a color similar to that of honey. Brown, green and yellow also occur. Four-rayed star chrysoberyl is also known to occur, but it is extremely rare and one ray is usually weaker.

The chatoyant band is chrysoberyl cat's-eye is produced by reflection of light from either needlelike rutile crystals or minute hollow tubes (negative crystals) arranged parallel to one another in profusion in a cabochon cut stone. An essential to a strong chatoyant effect, in addition to minimum size and a maximum number of inclusions, is high relief. The relief with which an object appears to stand out from its surroundings depends on the difference between the refractive index of the object and that of the material in which it is seen. The negative crystals, which usually seem to constitute the "silk" in a cat's-eye, give a high relief between the 1.75 of chrysoberyl and the 1.0 of air. Rutile needles also give a high relief (2.61-2.90 compared to 1.75).

The term CYMOPHANE (CY-moe-fane) has been used by some to refer to chrysoberyl that shows a broad sheen, rather than a sharp eye; by others to rough, material that could probably be cut to show an eye; and by some to be used interchangeably with cat's-eye. Since the term is subject to such widely divergent interpretations, and cat's-eye is clear in meaning, it is seldom used today.

When a good quality cat's-eye is held toward a concentrated light source with the chatoyant bang at right angles to the light, that half of the stone closest to the light will show the body color and the other half will appear quite milky. Assuming that the stone has a honey-colored body, the resultant appearance is often referred to as a "milk-and-honey" effect.

Another interesting effect that can be noted on a fine stone is the opening and closing of the eye when two overhead light sources are present. As the stone is revolved, the eye separates into two rays, divided by a clear area, and then comes together to form a single chatoyant band. If the cabochon is too steep, this pleasing effect may be absent.

Alexandrite Cat's-Eye

One of the most unusual of all gemstones is a cat's-eye that display a distinct to strong alexandrite type color change. It is truly a choice item for the connoisseur, since it is very rare and the combination of the two phenomena is striking. The usual relationship between the orientation of the needlelike inclusions and the best direction for the color change is such that the change is usually rather weak; the eye may be excellent, however.

Other Varieties

The other varieties of transparent chrysoberyl are not frequently seen in jeweler's stocks; some of them, however, are very attractive. Perhaps the best known is the one that resembles the light-colored peridot called chrysolite; in fact, this variety was often called "Oriental chrysolite". Such stones, usually Brazilian, could be described as greenish yellow to yellow-green of light to almost medium tone. Other varieties, particularly from Ceylon, are somewhat darker green, usually with a slightly brownish cast, resembling both green tourmaline and peridot. These stones have a clarity and beauty that make them very desirable. Brown to brownish-yellow to yellow-brown stones also may be very attractive, but they are seldom seen in dealers, stocks. Plate-blue chrysoberyls are encountered occasionally, but they are even rarer than alexandrite cat's-eye.

Formation & Sources of Chrysoberyl

Chrysoberyl is another of the numerous minerals that, in gem quality, is usually formed in pegmatite dikes; however, it is also formed in gneiss, mica-schist and granite. Associated with chrysoberyl in pegmatite dikes are such minerals as beryl, tourmaline and apatite, as well as the usual pegmatite assemblage of minerals. In gneiss or schist, it may be found with sillimanite, garnet or beryl.

In the various rocks in which it occurs, chrysoberyl is usually the result of the action of volatile constituents of a molten rock mass. The volatiles that were concentrated by crystallization of the least soluble minerals may bring about the formation of a pegmatite dike, or they may penetrate surrounding rocks along fractures or faults. In the latter instance, veins may be formed or isolated crystals of chrysoberyl and other minerals may form in the rocks that existed earlier as they are metamorphosed to gneisses or schist's.

Since chrysoberyl is a very hard and tough gemstone, it is a natural constituent of alluvial, deposits in areas in which pegmatite dikes are found.

Sources and Recovery Methods

Despite the fact that chrysoberyl is a comparatively rare mineral, it is found in a number of places in the world, in two or three of which fine gem qualities are mined. Gem material is almost invariably recovered from alluvial gravels, rather than being mined from primary deposits.

The most important source of both fine alexandrites and cat's-eyes is Ceylon. It is the only producer of alexandrite cats-eyes, although, of course, these are of minor importance because of their great rarity. At the present time, a significant producer of both cat's-eye and transparent chrysoberyl is Brazil, especially in the State of Minas Geraes. Other nearby gem-rich states are also producers. Russia produces both clear chrysoberyl and alexandrite from the area around Ekaterinburg (sverdlovsk), the daylight color of which tends slightly toward bluish green, rather than the yellowish green of the Ceylon type. Russian material is usually lower in quality, being not only less transparent but usually smaller in size. Gem chrysoberyl is also mined with other alluvial gem materials in Madagascar. It is one of the many minerals that occurs in and around Mogok, Burma; but, compared with most of the other production of that gem-rich area, it is rarely of the highest quality. Most of the Burma material tends towards very pale colors and to transparency, rather than toward cat's-eye material.

Chrysoberyl Jewelry

Cat's-eyes are always cut in cabochon, with the rare exception of the carved stones. Transparent material is usually faceted, either in the brilliant, step or mixed style. As explained in Importance of Cutting in Gemstones, proper proportions have a material effect on the value of cat's-eye: furthermore, it is important that fibrous material be oriented so that the hair like inclusions are at right angles to the length of the cabochon and in planes parallel to the girdle. Alexandrite must be oriented in a direction that makes both the red and green pair of the trichroic colors visible through the table; otherwise, it will exhibit too little change to merit the name alexandrite.

Cabochon material may be ground in the usual manner, using carborundum wheels and regular sanders for the preliminary operations. A wood tap with 1200-grit diamond dust is used for finer sanding, and polishing is accomplished with 6400-grit diamond dust on a wood wheel.

Transparent material is ground in the usual manner, using s diamond-impregnated laps. Polishing is accomplished on a tin lap, with Linde A power as the polishing agent; an alternate method is to use 6400-grit diamond dust on a tine or type-metal lap. Suggested angles are 37°for the crown and 42° for the pavilion.

Physical & Optical Properties of Chrysoberyl

Physical Properties

Chemical Composition Beryllium aluminate, expressed by the formula BeAIZ04.
Crystallographic Character Orthorhombic system. Habit; prismatic crystals. Cyclic, or radial, twinning and striations are frequent.
Hardness 8 ½
Toughness Excellent, except some nonphenomenal material that has been reported as only fair to good.
Cleavage Indistinct
Fracture Conchoidal
Specific Gravity 3.71 to 3.75; normal 3.73
Streak White
Characteristic Inclusions Chrysoberyl has no inclusions that can be regarded as characteristic of a significant proportion of specimens. Finger print inclusions, similar to those in corundum, are not uncommon. Silk like inclusions (negative crystals or rutile crystals), which cause the chatoyant band in cat's-eye, are also frequently encountered; however, they occur in a single direction, rather than in three sets of parallel needles, as in corundum, and are usually exceedingly fine. Yellow transparent stones only frequently show step like glide planes or twinning lines.

Optical Properties

Degree of Transparency Transparent to translucent
Luster Polished surfaces are vitreous to sub-adamantine: fracture surfaces are vitreous to greasy
Refractive index 1.746-1.755. These are the common readings. Each may vary about .004, but the birefringence is always. 008 to .009.
Birefringence .008to .009
Optic Character Biaxial positive
Pleochroism Yellow: distinct vary light yellow, greenish yellow, and colorless. Alexandrite: strong green, orange, and dark red purple
Dispersion 0.015
Phenomena Chatoyancy in cat's-eve and change of color in alexandrite. Four-rayed stars are known to occur, but they are exceedingly rare.
X-Ray Fluorescence Alexandrite may show a weak red glow; all others are inert.
Transparency to X-Rays Semitransparent
Ultraviolet Fluorescence Alexandrite may show a weak red glow under both long and short wavelengths. Exposure to short-wave ultraviolet light produces a yellow-green glow in greenish yellow chrysoberyl . All others are inert.
Color-Filter Reaction Alexandrite: red; brighter in artificial light. Yellow and green: no reaction.
Absorption Spectra Alexandrite: a strong doublet at 6805 and 6785 A.U. and weak lines at 6650, 6550 and 6450 A.U. Partial absorption of the yellow between 5800 and 6300, as well as three weak lines in the blue at 4765, 4730 and 4680 A.U. Also, general absorption in the violet. Yellow-green: a strong band in the bluish violet at 4450 A.U. The spectrum in alexandrite is caused by chromium; in the yellow green type, by ferric iron.
Effects Caused by heat Infusible before the blowpipe or the jeweler's torch
Effects Caused by Acids Not attacked
Effects Caused by irradiation No data available

Test and Identification of Chrysoberyl

In testing, there is not too much danger of confusing chrysoberyl with other materials that appear similar to the eye. The yellow and yellow-green varieties of sapphire and chrysoberyl may cause some confusion, but it is not difficult to separate them with the refractometer. Although the indices of chrysoberyl are given as 1.746-1.755 and those of corundum as 1.762-1.770, they do not cause confusion on the refractometer. Chrysoberyl is optically positive and corundum is optically negative; therefore, the constant reading of corundum and the nearly constant reading of chrysoberyl are at opposite ends of the two ranges.

In other words, chrysoberyl shows a difference between its low and middle indices of only 0.001, so that the usual reading appears to be approximately at the 1.746 figure, whereas the constant figure for corundum is 1.77. Thus, a casual reading in white light will usually show a difference in index of at least 0.02, a separation that is wide enough to be noted very easily. If there is any further reason for confusion, it can be allayed readily by the use of the polariscope to obtain an interference figure I since chrysoberyl is biaxial and sapphire is uniaxial. Also, the yellow chrysoberyl more often than not has the step like glide planes or twinning lines mentioned earlier.

Color changes may be seen in one variety of synthetic sapphire and a rather rare variety of synthetic spinel. Synthetic sapphire should not be mistaken for an alexandrite by anyone who has ever seen the natural stone, because the color changes from a grayish blue to a color in artificial light that more nearly resembles amethyst than the slightly brownish red for most alexandrite. However, so many have been sold either as "alexandrite" or "synthetic alexandrite" that a word or two on their separation seems indicated. Synthetic alexandrite like sapphire invariably contains very strong curved striae (growth lines). These lines may sometimes be noted with the unaided eye; they are almost always readily seen by an experienced observer under 10x. Since the indices of natural and synthetic sapphire are equal, an easily read refractive-index difference exists between synthetic sapphire and chrysoberyl. (Note: Natural sapphire with a slight color change is also encountered on occasion; again, however, the colors are not similar to those of alexandrite. It is more likely to be blue in daylight, instead of green, and its artificial-light color just seems to contain more violet than its daylight color.)

Distinguishing alexandrite visually from the better quality of synthetic spinel is more difficult, for the red to green color change is similar to that of chrysoberyl. However, spinel is nondichroic and has a distinctly lower R.I. (1.73). The exceedingly strong trichroic of alexandrite is a characteristic and a positive means of separation from materials that imitate it in appearance. The synthetic spinel imitation of alexandrite seems to be very difficult to obtain.

In appearance, it is possible to confuse peridot with chrysoberyl of comparable yellow-green color, but the high birefringence of peridot and its distinctly lower R.I. and S.G. make it possible to separate the two readily with the usual testing instruments.

The only other gemstones that show a cat's-eye similar to that of chrysoberyl are very fine varieties of quartz and an occasional tourmaline. Even at its best, the tourmaline bears only a slight resemblance to a poor quality of chrysoberyl. Very rarely, however, quartz cat's-eyes may be virtually indistinguishable in appearance from fine chatoyant chrysoberyl in this case, the much lower R.I. and S.G. of quartz make it possible to effect a separation very readily. Unknowingly, a New York colored-stone house once had a quartz cat's-eye among its stock of chrysoberyls for some time. While examining the stones and glancing at the weights listed, a gemologist realized that something was wrong, because two stones only a few papers apart were of nearly the same size but the weights listed differed by more than three carats. A quick check of R.I. by the spot method enabled him to identify the quartz among the more valuable chrysoberyls. A cat's-eye effect may also be encountered in fair to poor quality in beryl, feldspar, scapolite, diopside and other minerals.

A recently introduced imitation cat's-eye consists of a thin piece of ulexite (a white mineral with a fine, parallel fibrous appearance imparting to it a silky luster) sandwiched between two pieces of synthetic yellow sapphire. The bottom portion is rough ground and the top portion is polished. Although the "eye" is quite strong, this assembled stone is easily detectable because of the obvious plane at the girdle and the characteristic gas bubbles and complete transparency of the synthetic sapphire crown. In addition, under magnification, the cement used to join the three components is visible through the top of the stone. A glass imitation is also made that has a strong chatoyant band, but it can be separated readily from chrysoberyl by the obvious hollow tubes producing the band and by S.G. and R.I.

Andalusite, a rather rarely encountered mineral, has fairly strong green and brownish-red dichroism. This may be confused superficially with either the daylight or the night color of an alexandrite that does not show a strong color change, since in andalusite both green and reddish casts may be seen when the stone is examined. Andalusite has an R.I. in the range of topaz (near 1.62), and it is also much lower in S.G. than chrysoberyl.

Buying Tips for Chrysoberyl

Valuation

Intense, slightly greenish yellow or brownish yellow (honey yellow) are the most valuable colors of cat's-eye. Fine greens are a step lower in price than honey yellows. Darker brown body colors are slightly less valuable, and very pale yellow and very pale greenish yellow are even less valuable. Green stones are usually priced the same as the light brown colors. Opaque gray stones have relatively little value.

A narrow, well-defined chatoyant band exactly in the center of the stone is the most desirable. Silvery-white bands are preferred by some; others are partial to a golden-yellow eye. Less desirable are greenish or bluish-white eyes. The opaque gray stones usually have a bluish or bluish-gray eye. Whatever its color, however, it is important that the eye contrast sharply with the background color. For top value, the eye must open widely and close sharply; in addition, the milk-and-honey effect mentioned previously must be present. Fine qualities of alexandrite cat's-eye are rare and highly prized; even if they show only a fair color change, they are usually valued above other cat's-eyes.

Proportioning must also be considered before the relative quality of a cat's-eye can be determined. In an effort to conserve as much of the rough material as possible, lapidaries frequently cut cat's-eyes with such deep bases that they are difficult to set in a mounting properly. Conserving weight may be done in an attempt to increase the beauty of a stone; more often, however, it is done only to obtain a higher price, by multiplying the total weight by the per carat figure that the quality in the upper portion of the stone merits. Since most cat's-eyes are translucent to semitransparent, the depth of a stone is often of little importance, as far as the quality of the color or eye is concerned. Therefore, it is usually only necessary to retain enough weight below the girdle to give the stone a slightly rounded base that joins the girdle edges at a rather low angle, thus making it possible to secure the stone in a well-fitting bezel. The girdle of a stone with a deep base is usually too wide and rounded, to permit easy setting. Since weight retention is such a common practice, the method used by many dealers to establish a more realistic value is to consider the approximate weight of the stone as if it were to be re-cut to eliminate the excess pavilion weight. This figure, plus the per carat value based on the apparent quality of the crown, is then used to arrive at the more correct figure. However, there are some stones that are too transparent to have the body required for the milk-and-honey effect or the opening and closing of the eye. Such stones need the bulk below the girdle and merit a substantially lower price per carat.

The value of alexandrite is affected by imperfections and quality of cutting, of course, but primarily by the beauty of its change of color. It is usually implied that the closer the daylight color approaches the slightly bluish or yellowish green of fine quality emeralds and the more nearly the color under artificial light approaches red to purplish red of ruby, the more valuable the stone. However, neither color really should be likened to emerald or ruby, for the finest alexandrites bear little resemblance to either. The finest red (sometimes described as "columbine" or "raspberry"), is more comparable to that of a very fine garnet, and the green to an unusually intense tourmaline green. The variations from these perfection colors are wide and rarely are two stones similar, either in quality of the color change or in value. Although stones that have a dark grayish-green daylight color are less valuable, all alexandrites with a good change are rated among the more valuable gems. Large flawless crystals are rare; usually, they are so stratified and flawed that only small stones can be recovered. Ceylon stones are generally less flawed and often display a better artificial-light color, although beautiful stones have come from Russia. Compared with other gems, there is only a small quantity of alexandrites available at the present time.

Transparent chrysoberyl, other than alexandrite, is not frequently seen in jewelers, stocks, but it is hard, durable, and sometimes furnishes attractive stones. Green stones are particularly attractive. Price is related directly to the appeal of their color.

Buying Tips

Although chrysoberyl is not well known to the public, it enjoys a strong and seemingly growing demand among those who know and appreciate colored stones. Many retail firms have found that cat's-eyes "move" rather well, particularly to men; as a consequence, in recent years an increasing number of jewelers have stocked them. Some have found that one of the best means of selling this intriguing stone is to wear a ring themselves. One jeweler told that be had sold four from his hand within a period of a year, all sales of significant size. A glance at the price chart for chrysoberyl will show that a size able stone of fine quality can represent a sale of consequence. When an exceedingly fine cat's-eye can bring as much as $1000 per carat on the wholesale market, it is obvious that fifteen or twenty-carat stone mounted in platinum can command a very high figure. A growing demand and a limited supply is likely to continue to boost its value. In recent years, even medium grades have risen markedly in value as the demand has increased.

The prevalence of synthetic alexandrite like sapphire has called attention to the genuine alexandrite; as a result, its value has also increased materially in recent years. In chrysoberyl and all of its varieties, the jeweler has stone that is hard, beautiful and sufficiently rare to he highly desirable and worth promoting. This is not to suggest that alexandrites in any size and quality can be obtained at a moment's notice, but merely that they (as well as cat's-eyes) are available to a retailer who seeks them. Stones can be obtained on memo from most of the larger colored-stone houses. A jeweler who has a stone or two in stock and seeks a larger and finer stone for a customer or a smaller and poorer stone would thus be in a position to obtain a small selection within a reasonably short time.

Both alexandrite and cat's-eye are intriguing to most persons. A description by an alexandrite owner of his stone is often amusing, since it suggests that the stone has the ability to display almost every color of the spectrum. Although this is not true, it seems to leave that impression with its owner. Even synthetic alexandrite like sapphire, which is so often purchased overseas as a natural stone, seems to fascinate its owners with its unusual appearance and its property of changing color under different light sources. The opening and closing of the eye and the milk-and-honey effect are also a source of fascination to its owner. This is one of the reasons that either of these phenomenal varieties of chrysoberyl can be sold very effectively from.