1911 Encyclopædia Britannica/Chrysoberyl

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CHRYSOBERYL, a yellow or green gem-stone, remarkable for its hardness, being exceeded in this respect only by the diamond and corundum. The name suggests that it was formerly regarded as a golden variety of beryl; and it is notable that though differing widely from beryl it yet bears some relationship to it inasmuch as it contains the element beryllium. In chrysoberyl, however, the beryllium exists as an aluminate, having the formula BeAl2O4, or BeO·Al2O3. The analysis of a specimen of Brazilian chrysoberyl gave alumina 78·10, beryllia 17·94, and ferric oxide 4·88%. The typical yellow colour of the stone inclines in many cases to pale green, occasionally passing into shades of dark green and brown. The iron usually present in the mineral seems responsible for the green colour. Chrysoberyl is often mistaken by its colour for chrysolite (q.v.), and has indeed been termed Oriental chrysolite. In its crystalline forms it bears some relationship to chrysolite, both crystallizing in the orthorhombic system, but it is a much harder and a denser mineral. As the two stones are apt to be confounded, it may be convenient to contrast their chief characters:—

  Chrysoberyl. Chrysolite.
Hardness 8·5 6·5 to 7
Specific Gravity 3·65 to 3·75  3·34 to 3·37
Chemical Composition  BeAl2O4. Mg2SiO4.

Chrysoberyl is not infrequently cloudy, opalescent and chatoyant, and is then known as “cymophane” (Gr. κῦμα, a “cloud”). The cloudiness is referable to the presence of multitudes of microscopic cavities. Some of the cymophane, when cut with a convex surface, forms the most valuable kind of cat’s-eye (see Cat’s-Eye). A remarkable dichroic variety of chrysoberyl is known as alexandrite (q.v.).

Most chrysoberyl comes from Brazil, chiefly from the district of Minas Novas in the state of Minas Geraes, where it occurs as small water-worn pebbles. The cymophane is mostly from the gem-gravels of Ceylon. Chrysoberyl is known as a constituent of certain kinds of granite, pegmatite and gneiss. In the United States it occurs at Haddam, Conn.; Greenfield Centre, near Saratoga Springs, N.Y.; and in Manhattan island. It is known also in the province of Quebec, Canada, and has been found near Gwelo in Rhodesia.  (F. W. R.*)