E MERALD is now but a general trade designation for various green precious and semi-precious stones and not, in the jewelry trade, the specific term of any gem mineral. Beryl, of the accepted green emerald hue, is the true or standard emerald. In the view of the mineralogical experts of the United States National Museum, recognition is accorded to five other varieties of " emeralds "; they are: Brazilian-tourmaline, Congo-dioptase, Evening-olivine, Oriental-corundum, and Uralian-gar-net. The green beryl, excepting in its colour, is the same mineral as aquamarine, golden, and other variously coloured beryls. One of the rarest of gems is a flawless emerald-hued beryl.
The crystallisation of the beryl is in the hexagonal system, usually long, and often having the prism faces more or less deeply striated