In hardness the varieties of Beryl differ little from quartz, the hardness being 7.5 to 8 in the scale of which quartz is 7. 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; the specific gravity of quartz being 2.65. The varieties of Beryl are therefore relatively light as compared with other gems.
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 of Natural History and in the United States National Museum are exhibited single crystals also of great size. That in Boston is three and one-half feet long by three feet wide and weighs several tons. That in the National Museum weighs over six hundred pounds.
None of these crystals are of a high degree of purity or transparency, but the crystal planes at least of the prisms are well developed.
Beryl crystals have no marked cleavage except a slight one parallel with the base. Where broken, the surface shows what is called conchoidal fracture, i. e. it exhibits little rounded concavities and convexities resembling a shell in shape.
The mineral is quite brittle. Some emeralds even have the annoying habit of breaking of their own accord soon after removal from the mine. This can be prevented by warming them gradually before exposing them to the heat of the sun or other sudden heat.
Beryl and its varieties, like tourmaline, are dichroic, i. e. the stones exhibit different colors when viewed in different directions. This dichroism can sometimes be observed by the naked eye, but often not without the aid of the instrument known as the dichroscope. When seen it furnishes a positive means of distinguishing a true stone from any glass imitations.
The varieties of Beryl have none of the brilliancy of the diamond and therefore depend wholly on their body colors and their lustre for their beauty and attractiveness. Fortunately they exhibit these qualities as well by artificial light as by daylight.
