BERYL, a mineral, among the varieties of which are found two of the moist beautiful and costly gems with which we are acquainted, namely, the Emends/ and the Precious Beryl.
They belong to the rhombobedral system of crystallisation, usually occurring in regular hexagonal prisms which occur variously modified, sometimes by the truncation of the lateral edges of the prism, at other times by the simple truncation of the terminal edges; but the prism is sometimes terminated in a much more complicated manner, of which a remarkable instance has presented itself in a crystal in the possession of Professor Nainnann of Freiberg, who has observed in them the faces of no less than six other forms of the rhembohedral system. Its general aspect is always that of a hexagonal prison, and when the terminal edges are modified there will generally be found a plane inclined to the lateral planes of the prism under an angle of 110* 58'.
The crystals admit of cleavage in the four directions parallel to the faces of the regular prism, that parallel to the terminal plane being perfect, the others imperfect and more difficult to be obtained. The fracture is eonchoidal and uneven ; the lustre is vitreous, and it possesses various degrees of transparency. According to Mohs the hardness varies from to 8, the specific gravity from to 2/32. The following are its chemical characters before the blowpipe, as stated by Berzelius.
Alone it is not easily acted upon, but when thin fragments are for a long time submitted to a powerful flame the edges become rounded, and a colourless vesicular scoria is produced. The transparent varieties become milky.
With borax it forms a clear and generally colourless glass, which effect is also produced by soda. With the phosphor salt it is with difficulty dissolved without the formation of a silicious skeleton.
Of this mineral we possess several analyses, of which the following are three : the first being an Emerald from Peru, by Klaproth ; the second a Beryl from Siberia, by the same chemist; and the third a Beryl from Broddbo, near Fahlun, in Sweden : Beryl. Beryl.
Emerald. Siberia. Broddbo.
Silica 68'50 68'35 Alumina . . . . 1515 Glucina • 13'13 Oxide of Iron . . Oxide of Columbium 0/7 Oxide of Chromium 0'00 Lime 0.00 This species contains several varieties, of which the two known among lapidaries under the name of Emerald and Aquamarine, or Precious Beryl, are the most worthy of attention. These varieties, though distinguished by some mineralogists as forming distinct species, differ however only in colour, the term Emerald being applied to those possessing the peculiar rich deep green so well known as the emerald-green while all the other varieties are comprehended under the name of Beryl Beryl; those which are clear, transparent, and possess a good colour, present various shades of sky-blue or Mountain-green, being the Aquamarine or Precious BeryL The colour of the Emerald is attributed to the small quantity of green oxide of chromium which has been found in the specimens from Peru; while the varieties in the tints of Beryl may be considered to be produced by admixtures of the oxides of iron, the yellow being the colour of the peroxides of iron, and the mountain-green and the various shades of blue being the effect of varying quantities of the protoxide, to the presence of which the common bottle-glass owes its tint.
The following localities produce the finest Emeralds :—The mines in the Tunca Valley, situated in the mountains between New Granada and Popayan, and not far from the town of Santa Fti de Bogota, where, according to Humboldt, they are found in veins traversing clay slate, hornblende-slate, and granite ; the Heubach Valley, in the dis trict of Pinzgau, Salzburg, where they occur imbedded in mica-slate, and are inferior in colour to those from Peru • varieties have also been lately found in some old mines in Mount Zabarah, in tipper Egypt, from which spot the ancients are supposed to have derived their emeralds.