Crystals belonging to this system are also doubly refractive and uniaxial, the direction of single refraction being parallel to the fourth axis mentioned above, and therefore also parallel to the prism edge. Hence deeply coloured tourmaline, which strongly absorbs the ordinary ray, must be cut with the table-facet parallel to the edge of the prism. [Illustration: FIG. 11.—Relation of the two directions of single Refraction to the Axes in an Orthorhombic Crystal.] 4. _Orthorhombic._—Such crystals can be referred to three axes, which are mutually at right angles, but in which each of the directional characters are different. The crystals are usually prismatic in shape, one of the axes being parallel to the prism edge. Topaz, peridot, and chrysoberyl are the most important species crystallizing in this system. Crystals belonging to this system are doubly refractive and biaxial, _i.e._ they have two directions of single refraction (cf. p. 45). The three axes _a_, _b_, _c_ (Fig. 11) are parallel respectively to the two bisectrices of the directions of single refraction, and the direction perpendicular to the plane containing those directions. 5. _Monoclinic._—Such crystals can be referred to three axes, one of which is at right angles to the other two, which are, however, themselves not at right angles. Spodumene (kunzite) and some moonstone crystallize in this system. Crystals belonging to this system are doubly refractive and biaxial, but in this case the first axis alone is parallel to one of the principal optical directions. 6. _Triclinic._—Such crystals have no edges at right angles, and the optical characters are not immediately related to the crystalline form. Some moonstone crystallizes in this system. [Illustration: FIG. 12.—Twinned Octahedron.] Crystals are often not single separate individuals. For instance, diamond and spinel are found in flat triangular crystals with their girdles cleft at the corners (Fig. 12). Each of such crystals is really composed of portions of two similar octahedra, which are placed together in such a way that each is a reflection of the other. Such composite crystals are called twins or macles. Sometimes the twinning is repeated, and the individuals may be so minute as to call for a microscope for their perception. A composite structure may also result from the conjunction of numberless minute individuals without any definite orientation, as in the case of chalcedony and agate. So by supposing the individuals to become infinitesimally small, we pass to a glass-like substance. It is often a peculiarity of crystals of a species to display a typical combination of natural faces. Such a combination is known as the habit of the species, and is often of service for the purpose of identifying stones before they are cut. Thus, a habit of diamond and spinel is an octahedron, often twinned, of garnet a dodecahedron, of emerald a flat-ended hexagonal prism, and so on.