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.

 

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