BERYL: EMERALD and AQUAMARINE
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The mineral species beryl includes, besides the emerald and the aquamarine, other precious stones of less importance, which are referred to generally by jewellers as beryl. The different varieties of beryl differ only in colour; their other characters are identical, just as ruby and sapphire are mere colour varieties of the mineral species corundum. It will be convenient to consider the specific characters before passing to a more detailed description of each colour-variety.
The oxide alumina, which enters so largely into the composition of corundum, spinel, and chrysoberyl, is also present in beryl, but in smaller amount and in combination with silica and beryllium. The oxide of beryllium, so called on account of its presence in beryl, is also a constituent of chrysoberyl. Beryl is thus a silicate of the metals aluminum and beryllium, the chemical composition of which is expressed by the formula 3BeO.Al2O3.6SiO2, and the percentage composition by, silica (SiO2) = 66.84, alumina (Al2O3) = 19.05, beryllia (BeO) = 14.11.
In several analyses of this mineral the presence, in small amounts, of water, iron, alkalis, chromic oxide, and other substances have been determined. The result of the analysis of this stone together with an analysis of aquamarine from Adun-Chalon, in Siberia, is given below. Chromic oxide, which is absent from this specimen of aquamarine and exists as a trace in the emerald, is sometimes present to the extent of 3 per cent.
Beryl crystallizes in the hexagonal system. The crystals (Fig. a to e) are usually rather long, six-sided prisms with smooth faces, terminated in many cases, and nearly always in emerald, by a single plane at right angles to the faces of the prism (Fig. a), this being known as the basal plane. Not infrequently the edges of the hexagonal prism are truncated by the faces of a second hexagonal prism, and these again by a twelve-sided prism; the resulting form, though in reality a prism, bounded by many small faces (Fig. d), has the appearance of a longitudinally-striated cylinder. For this reason the prism faces of beryl are usually striated in the direction of their length, that is, parallel to their mutual intersections. Moreover, in many cases, the crystals are terminated not only by the basal plane but also by six-sided and twelve-sided pyramids in combination with the prism. Fig. b shows a hexagonal pyramid of the second order, and Fig. c a hexagonal pyramid of the first order, while in Fig. e there are two hexagonal pyramids of the first order, one of the second order, and a dihexagonal pyramid in combination with a hexagonal prism and the basal plane. These more complicated crystals are more characteristic of aquamarine.
The crystals are either attached to the matrix by one end, in which case they often form beautiful druses, or they are embedded in it, and are then developed regularly in all directions. In the former case, the free end alone bears regular crystal-faces, while in the latter both ends are developed; these terminal faces are, however, sometimes small and irregular.
The cleavage of beryl is not an important character, crystals of this mineral cleaving only indistinctly in certain directions. The mineral is brittle, and its fracture conchoïdal.
With respect to its hardness it stands a little above quartz, but below topaz, the degree of its hardness being represented on the scale by H = 7 1/2 this is rather low for a precious stone, and beryl is, in fact, one of the softer of the more valuable gems. The different varieties of the mineral show small differences in hardness among themselves, the Colombian emerald being, for example, a little softer than the Siberian aquamarine. Although the hardness of this stone is not great, it is sufficient to render it susceptible of a fine polish, which, however, is not retained for as long as is the case with harder stones. The specific gravity of beryl, like its hardness, is rather low, its mean value being 2.7 g/cm3. That of precious beryl varies between 2.67 and 2.75 g/cm3, being always slightly higher than the specific gravity of quartz (2.65 g/cm3). The specific gravity of certain specimens of emerald from Muzo has been found to be 2.67 g/cm3, while for Siberian aquamarine the values 2.68 to 2.75 g/cm3 are given. Beryl will thus always float in methylene iodide, and when pushed beneath the surface quickly rises again; in liquid No.4 with the specific gravity of quartz it slowly sinks. This character affords a ready means by which it may be distinguished from certain stones of similar appearance.
With the exception of hydrofluoric acid, acids do not attack beryl. When a fragment is heated before the blowpipe it becomes white and cloudy, and fuses, only with difficulty, at the edges to a white, blebby glass.
Different beryls differ greatly in appearance, especially in respect to color and transparency, some being cloudy and opaque, others beautifully clear and transparent; all possible gradations between these two extremes are known. The opaque variety, known as common beryl, usually occurs as crystals in coarse-grained granite. Such crystals have been known to measure 6 feet in length, and to weigh 2 ½ tons; they are useless, however, for gems since, besides being opaque, they are usually of an unpleasing yellowish or greenish white color. Only the transparent or semi-transparent precious beryl is used for cutting as gems; this is usually of a beautiful colour, often green or blue, but sometimes yellow. All varieties of beryl have the common vitreous lustre.
In correspondence with its crystalline form, all hexagonal crystals being birefringent, beryl is doubly refracting, only, however, to a small extent, for the greater and lesser indices of refraction for the same colour differ but slightly. Its refraction is also small. In the case of emerald from Muzo the greater and lesser refractive indices have been determined to be 1.584 and 1.578, and in the case of Siberian aquamarine 1.584 and 1.576. The dispersion is also small, the refractive coefficients given by the same crystal for differently coloured rays of light differing but slightly. This may be seen by comparing the refractive indices of a crystal of beryl for red, yellow, and green light which are given below:
It follows, then, that scarcely any play of prismatic colours, such as is characteristic of the diamond, is seen in beryl; its beauty depends mainly on its strong lustre, and on its fine body-colour. There is a certain amount of variety in this latter character, but very much less than in corundum. Green and bluish-green beryl is most common, yellow rather less so, pale red and water-clear stones rare. Different varieties of beryl are distinguished by their colour; bright grass-green beryl being known as emerald. Other varieties, which are always of a pale colour, are referred to as precious or noble beryl. Of these light-coloured varieties, the pale blue, bluish-green, or yellowish-blue, is distinguished as aquamarine, the yellowish-green as "aquamarine-chrysolite", while jewellers know the yellow variety as beryl, and when of a pure golden-yellow as golden beryl. Of all these varieties the emerald is by far the most important as a precious stone, ranking, indeed, with the costliest of gems; aquamarine is also much used, while the other varieties are of less importance.
All transparent beryls, whatever their colour, if not too pale, are distinctly dichroïc, differences in colour can often be observed with the naked eye, and with the dichroscope are, as a rule, unmistakable. On account of this property, it is possible, therefore, to distinguish genuine beryl from coloured glass imitations, or from other gemstones, which it may resemble in appearance.
The characters by which the different varieties of beryl are distinguished must now be considered. These varieties, which differ from each other principally in colour, are by no means equally valuable as gems, the emerald being by far the most costly.
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