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GEMS AND PRECIOUS STONES. 43
Simple Forms of the Tetragonal System.
When the zircon is found crystallized it can hardly be mistaken for any other gem, as it crystallizes in this system.
In
this system there are three axes, two of which are equal in length, the
other may be either longer or shorter than these, this is the vertical
or principal axis, the two equal ones are the lateral. All three axes
are at right angles to each other.
Tetragonal pyramid.—This
figure is bounded by eight isosceles triangles, these being more obtuse
or acute as the principal axis is shorter or longer. The base of the
pyramid is a square, and the figure is a combination of two of these
pyramids as in the octahedron. Iu the holohedral forms there are two
sets of figures depending on the position of the lateral axes; in the
first they join the solid tetragonal angles, in the other, the centre
of the lateral edges. It is easily seen that, if the faces of both
these are developed in the same crystal, we haye eight angles, and our
crystal will have become a diteiragonal pyramid—which figure is bounded by sixteen isosceles triangles, the base of the pyramids being octagonal.
Tetragonal prism.—This
form is bounded by four equal rectangles, having two square basal
planes to close the figure. There are also two forms in this, as in the
pyramid, based upon the position of the lateral axes ; in one they join
the centre of the rectangles, in the other the vertical edges. This is
a common form of the zircon, but the figure is usually surmounted by
pyramids instead of the basal planes. If faces of both orders are
developed in the same crystal we obtain a ditetragonal prism—which figure is bounded by eight rectangular planes, and the basal plane will, of course, be octagonal.
There are two hemihedral forms, the tetragonal sphenoid, having four faces formed from the tetragonal pyramid by the development of alternate faces, and the tetragonal scalenohedron, having
eight faces formed by the development of vertical alternate pairs of
faces of the ditetragonal pyramid. These two forms are rarely met with
except in combinations.
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CHRYSOBERYL.
Crystalline system—Rhombic.
Hardness —8 5.
Specific gravity—3'5—381.
Lustre—Vitreous.
Cleavage—Parallel to the brachydome, less distinct parallel to the brachy-pinakoid.
Composition—Alumina and Glucina (Beryllia), BeO, Al203=Alumina 80'2, Glucina 198 per cent.
This gem has several varieties, known by characteristic names ; as the cymophane, which embraces the true cat's-eye, the Alexandrite, named
after the Emperor of Russia, and the hard specimens called " Oriental
chrysolite" by jewellers ; these are all chrysoberyls, and although
differing in hue and physical appearance from each other, contain no
essential differences in composition. The hardness of chrysoberyl,
together with its lustre and brilliancy, make it a desirable stone for
ornamental purposes, as it has a hardness nearly equal to the sapphire,
and its brilliancy approaches that of the diamond. It has a good range
of colour, but the yellow and greenish-yellow specimens predominate,
although white stones are occasionally found.
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