The Beryl Family
Emerald 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-garnet. 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 vertically. The specific gravity of the transparent flawless beryl is 2.73, usually 2.69 to 2.70; hardness, 7.5 to 8; brittle; cleavage indistinct; fracture uneven to conchoidal; lustre vitreous, sometimes resinous. Beryl colours include emerald green to pale green, pale blue, pale yellow, honey, wine and citrine yellow, white, and pale rose-red. Pleochrism is unusually distinct, sometimes strong, in the emerald especially, which through the dichroiscope reveals two different shades of green.
Beryl includes the emerald, aquamarine, goshenite, and davidsonite. The differences are principally in colour.
Beryl is a silicate of the metals aluminium and beryllium, containing the oxide alumina in small amount, which is, however, a more important constituent in corundum, spinel, and chrysoberyl. There is some variation in beryl from different localities; the chemist Lewy, who analysed the beautiful emerald beryl that is found at Muzo in Colombia, South America, found: silica, 67.85; alumina, 17.95; beryllia, 12.4; magnesia, 0.9; soda, .07; water, 1.66; and organic matter 0.12, besides a trace of chromic oxide. An analysis of a specimen of aquamarine from Adun-Chalon in Siberia by Penfield resulted in: silica, 66.17; alumina, 20.39; beryllia, 11.50; ferrous oxide, 0.69; soda, 0.24; water, 1.14, and a trace of lithia.
The only acid which will attack beryl, so far as has been discovered, is hydrofluoric acid. Before the blowpipe beryl becomes white, cloudy, and fuses, but only with difficulty, at the edges to a white blebby glass.
Beryl, like all other hexagonal crystals, is bi-refringent, but only to a small extent. The beauty of beryl, therefore, depends not upon a play of prismatic colours, but upon unusually strong lustre and a fine body-colour. The bright grass-green beryl is the emerald; the pale varieties are styled precious or noble beryl. Aquamarine is pale-blue, bluish-green, or yellowish-blue; the yellowish-green variety is called aquamarine-chrysolite; jewellers call the yellow variety beryl and the pure golden-yellow golden beryl. The dichroism of all transparent varieties of beryl can often be discerned with the eye unaided by the dichroiscope; this property usually suffices to clearly distinguish beryl from any imitations. A curious characteristic of the emerald beryl is that its colour is by no means always uniformly distributed through the body of the stone; the different coloured portions may occur in layers or irregularly; when in layers the layers are usually perpendicular to the faces of the prisms.