George Frederick Herbert Smith

CRYSTALLINE FORM

WITH the single exception of opal, the whole of the principal mineral species used in jewellery are distinguished from glass and similar substances by one fundamental difference: they are crystallized matter, and the atoms composing them are regularly arranged throughout the structure.

The words crystal and glass are employed in science in senses differing considerably from those in popular use. The former of them is derived from the Greek word ?????, meaning ice, and was at one time used in that sense. For instance, the old fourteenth-century reading of Psalm cxlvii. 17, which appears in the authorized version as “He giveth his ice like morsels,” ran “He sendis his kristall as morcels.” It was also applied to the beautiful, lustrous quartz found among the eternal snows of the Alps, since, on account of their limpidity, these stones were supposed, as Pliny tells us, to consist of water congealed by the extreme cold of those regions; such at the present day is the ordinary meaning of the word. But, when early investigators discovered that a salt solution on evaporation left behind groups of slender glistening prisms, each very similar to the rest, they naturally—though, as we now know, wrongly—regarded them as representing yet another form of congealed water, and applied the same word to such substances. Subsequent research has shown that these salts, as well as mineral substances occurring with natural faces in nature, have in common the fundamental property of regularity of arrangement of the constituent atoms, and science therefore defines by the word crystal a substance in which the structure is uniform throughout, and all the similar atoms composing it are arranged with regard to the structure in a similar way.

The other word is yet more familiar; it denotes the transparent, lustrous, hard, and brittle substance produced by the fusion of sand with soda or potash or both which fills our windows and serves a variety of useful purposes. Research has shown that glass, though apparently so uniform in character, has in reality no regularity of molecular arrangement. It is, in fact, a kind of mosaic of atoms, huddled together anyhow, but so irregular is its irregularity that it simulates perfect regularity. Science uses the word glass in this widened meaning. Two substances may, as a matter of fact, have the same chemical composition, and one be a crystal and the other a glass. For example, quartz, if heated to a high temperature, may be fused and converted into a glass. The difference in the two types of structure may be illustrated by a comparison between a regiment of soldiers drawn up on parade and an ordinary crowd of people.

The crystalline form is a visible sign of the molecular arrangement, and is intimately associated with the directional physical properties, such as the optical characters, cleavage, etc. A study of it is not only of interest in itself, but also of great importance to the lapidary who wishes to cut a stone to the best advantage, and it is, moreover, of service in distinguishing stones when in the rough state.

The development of natural faces on a crystal is far from being haphazard, but is governed by the condition that the angles between similar faces, whether on the same crystal or on different crystals, are equal, however varying may be the shapes and the relative sizes of the faces (Fig. 1), and by the tendency of the faces bounding the crystal to fall into series with parallel edges, such series being termed zones. Each species has a characteristic type of crystallization, which may be referred to one of the following six systems:—

1. Cubic.—Crystals in this system can be referred to three edges, which are mutually at right angles, and in which the directional characters are identical in value. These principal edges are known as axes. Some typical forms are the cube (Fig. 2), characteristic of fluor; the octahedron (Fig. 3), characteristic of diamond and spinel; the dodecahedron (Fig. 4), characteristic of garnet; and the triakisoctahedron, or three-faced octahedron (Fig. 5).

All crystals belonging to this system are singly refractive.

2. Tetragonal.—Such crystals can be referred to three axes, which are mutually at right angles, but in only two of them are the directional characters identical. A typical form is a four-sided prism, mm, of square section, terminated by four triangular faces, p (Fig. 6), the usual shape of crystals of zircon and idocrase.

Crystals belonging to this system are doubly refractive and uniaxial, i.e. they have one direction of single refraction (cf. p. 45), which is parallel to the unequal edge of the three mentioned above.