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.
It is one of the most interesting and remarkable features connected with crystallization that the composition and the physical characters—for instance, the refractive indices and specific gravity—may, without any serious disturbance of the molecular arrangement, vary considerably owing to the more or less complete replacement of one element by another closely allied to it. That is the cause of the range of the physical characters which has been observed in such species as tourmaline, peridot, spinel, etc. The principal replacements in the case of the gem-stones are ferric oxide, Fe2O3, by alumina, Al2O3, and ferrous oxide, FeO, by magnesia, MgO.
A list of the principal gem-stones, arranged by their chemical composition, is given in [Table I] at the end of the book.
CHAPTER III
REFLECTION, REFRACTION, AND DISPERSION
IT is obvious that, since a stone suitable for ornamental use must appeal to the eye, its most important characters are those which depend upon light; indeed, the whole art of the lapidary consists in shaping it in such a way as to show these qualities to the best advantage. To understand why certain forms are given to a cut stone, it is essential for us to ascertain what becomes of the light which falls upon the surface of the stone; further, we shall find that the action of a stone upon light is of very great help in distinguishing the different species of gem-stones. The phenomena displayed by light which impinges upon the surface separating two media[1] are very similar in character, whatever be the nature of the media.