Chrysoberyl gemstone

by Manisha G

chrysoberyl group of gemstonesChemical Composition : Beryllium aluminate.

Crystal Characteristics : Orthorhombic system. Interpenetrant or cyclic twinning is common, giving rise to complicated habits having a hexagonal appearance. Single crystals are rare, but when found have a habit similar to that of peridot.

Varieties : Pale lemon-yellow to golden-yellow, brown and green; grass green to brownish-green with colour change to red or brown Alexandrite. Cloudy yellowish, greenish to brownish-green showing metamerism Cymophane chatoyant effect-chrysoberyl Cat’s-eye.

Physical Properties

Cleavage : Two directions parallel to dome form, not very distinct.

Hardness : 8.5 S.G. 3.71 to 3.72. Lustre : vitreous; Refraction: Double biaxial positive R.I. 1.745 – 1.754, Birefringence : 0.009

Dichroism : Yellow chrysoberyl weak. Alexandrite : Strong, trichroism : red, orange and green.

Other Important points :

Chatoyancy, characteristic of cymophane has been dealt with previously. Generally the greater the curvature of the stone, the sharper the eye effect produced; stones with a very sharp streak of light are highly prized. Cymophane is harder and heavier than quartz cat’s eye. Care must be taken in distinguishing between yellow or green chrysoberyls and corundums of the same shades but the R.I.s and S.G. are lower than corresponding constants of corundum.

Alexandrite : Colour is due to a small quantity of chromic oxide. Green by daylight and red by artificial incandescent light (not fluo­rescent tubes), a fact ascribed to the broad absorption bands seen in the absorption spectrum. These absorption regions are in the blue and yellow parts of the spectrum so that the light which emerges from the stone consists largely of green and red rays. If the stone is examined in light rich in the shorter wavelengths, it appears green, whilst if the light is rich in the longer wavelengths, the stone appears red. The colour change is less marked in stones containing iron as an impurity.

In addition to the broad absorption regions mentioned above, the spectrum of alexandrite shows narrow lines in the red and some­times in the blue, due to chromium, not unlike those in ruby. Spectrum – 4680, 4730, 6450, 6550, 6650, 6785 & 6803 A doublet. Yellow and green chrysoberyls show a broad absorption band centred at 4440A in the blue-violet, often seen as a cut-off, by which they may be identified, but care must be taken to avoid confusion with similar bands in green sapphire.

Cut: Transparent chrysoberyl, brilliant, step or mixed. Cat’s-eye is always cut in cabochon.

Detection : The R.l. of chrysoberyl is close to corundum but there is no actual overlap and the small difference is quite clearly detectable on the refractometer. The dark green stones without colour change may be mistaken for green sapphire. But the latter will show a series of three dark bands in the blue of their spectrum while the chrysoberyl will show only one. Yellow stones will not be so easily confused, but the chrysoberyl will again give the absorption in the blue while the sapphire will not. A few greenish natural sapphires with a noticeable colour change to reddish brown are also known, but the colour change is not as sharp as Alexandrite and the spectrum is quite different, besides the R.I. and S.G. are also high.

“Synthetic alexandrites” are generally poor imitations of the real stone. They consist of synthetic corundum with incorrect colour change, and also very simply detected by normal tests if their nature is not already too obvious at first glance. Synthetic spinels have also been made which show a better colour change from dark green to red. Their R.I. is that of synthetic spinel (1.727).

Cat’s-eyes can be differentiated from quartz cat’s-eyes by their greater specific gravity, the presence of the chrysoberyl line in the blue of the spectrum and visually by the fact that the “eye” is much narrower and sharper line than in the other gems. In difficult cases a distant vision test for the refractive index will show that the value is either around 1.75, a sufficiently wide disparity in values to make the identity of the stone quite clear. Yellow chatoyant tourmaline, Beryl and Apatite of similar appearance to the chrysoberyl cat’s-eye may also be found. The R.I. and S.G. tests will detect these.

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