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BERYL
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(EMERALD, AQUAMARINE, ETC.)
This mineral species includes a number of varieties which are highly valued as gems. These are, besides beryl itself, the gems emerald, aquamarine, and golden beryl. Of these, emerald is dark-green beryl, aquamarine bluish-green, or greenish-blue beryl, and golden beryl, yellow beryl. . Chrysoberyl is not a variety of beryl.
While these varieties of beryl all differ in color, they are the same mineral, and are practically identical in composition, hardness, and other properties. In composition, they are a silicate of aluminum and glucinum, the percentages being,for normal beryl: 67 per cent of silica, 19 per cent of alumina, and 14 per cent of glucina.
The beautiful green color of the emerald is probably due to a small quantity of chromium which it usually contains, though some authorities believe organic matter to be the coloring ingredient. To what substance the other varieties of the species owe their color is not known.
In hardness the varieties of beryl differ little from quartz, the hardness being 7.5 to 8. They are somewhat inferior, therefore, to such gems as topaz, sapphire, and ruby in wearing qualities, although hard enough for ordinary purposes.
The specific gravity of beryl is also about like that of quartz, ranging from 2.63 to 2.80. It is, therefore, relatively light as compared with other gems. Beryl is practically infusible, and is not attacked by acids.
Beryl crystallizes in the hexagonal system. It usually occurs as six-sided prisms, commonly terminated by a single flat plane, but sometimes by numerous small planes, giving a rounded effect, and occasionally by pyramidal planes, which cause the prism to taper to a sharp point.
The crystals sometimes grow to enormous size, exceeding those of any other known mineral. Thus, one found in Grafton, New Hampshire, was four and one-quarter feet in length, and weighed two thousand nine hundred pounds. Another in the same locality is estimated to weigh two and one-half tons. In the Museum of the Boston Society
98
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