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.



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