Quartz belongs to the hexagonal system of crystalline symmetry, and crystallizes in the familiar six-sided prisms terminated by six inclined, often triangular, faces (Fig. 79); twins are common, though they are not always obvious from the outward development. In accordance with the symmetry the refraction is double, and there is one direction of single refraction, namely, that parallel to the edge of the prism. The ordinary refractive index has the value 1·544, and the extraordinary 1·553, and since the latter is the greater, the sign of the double refraction is positive. The double refraction is small in amount, but is large enough to enable the apparent doubling of certain of the opposite edges of a faceted stone to be perceptible when viewed with a lens through the table-facet. The dichroism of the deep-coloured stones is quite distinct. Quartz has only about the same amount of colour dispersion as ordinary glass, and lacks, therefore, ‘fire.’ The application of strong heat tends, as usual, to weaken or drive off the colour. Thus the dense smoky-quartz found in Spain, Brazil, and elsewhere is converted into stones of a colour varying from light yellow to reddish brown according to the amount and duration of the application. In the case of amethyst the colour is changed to a deep orange, or entirely driven off if the temperature be high enough. Its density is very constant, varying only from 2·654 to 2·660; the purest stones are the lightest. To it has been assigned the symbol 7 on Mohs’s scale of hardness. To physicists quartz is one of the most interesting of minerals because of its power of rotating, to an extent depending upon the thickness of the section, the plane of polarization of a beam of light traversing it in a direction parallel to the prism edge. It appears, moreover, from a study of the pyro-electric and general physical characters, that its molecular structure has a helical arrangement, which, like all screws, may have a right- or left-handed character. Amethyst is, in fact, invariably composed of separate twin individuals, alternately right- and left-handed; in some remarkable crystals the section at right angles to the prism edge is composed of triangular sectors, alternately of different hands and of different tints—purple and white. To the twinning is due the rippled fracture and the feathery inclusions so characteristic of amethyst.