Certain corundums when viewed in the direction of the crystallographical axis display six narrow lines of light radiating at angles of 60° from a centre in a manner suggestive of the conventional representations of stars. Such stones are consequently known as asterias, or more usually star-stones—star-rubies or star-sapphires, as the case may be, and the phenomenon is called asterism. These stones have not a homogeneous structure, but contain tube-like cavities regularly arranged at angles of 60° in planes at right angles to the crystallographical axis. The effect is best produced when the stones are cut _en cabochon_ perpendicular to that axis. Chatoyancy is a somewhat similar phenomenon, but in this case the fibres or cavities are parallel to a single direction, and a single broadish band is displayed at right angles to it. Cat’s-eyes, as these stones are termed, are cut _en cabochon_ parallel to the fibres. The true cat’s-eye (Plate XXIX, Fig. 1) is a variety of chrysoberyl, but the term is also often applied to quartz showing a similar appearance. The latter is really a fibrous mineral, such as asbestos, which has become converted into silica. The beautiful tiger’s-eye from South Africa is a silicified crocidolite, the original blue colour of which has been altered by oxidation to golden brown. Recently tourmalines have been discovered which are sufficiently fibrous in structure to display an effective chatoyancy. The milky sheen of moonstone (Plate XXIX, Fig. 4) owes its effect to reflections from twin lamellæ. The wonderful iridescence which is the glory of opal, and is therefore termed opalescence, arises from a structure which is peculiar to that species. Opal is a solidified jelly; on cooling it has become riddled with extremely thin cracks, which were subsequently filled with similar material of slightly different refractivity, and thus it consists of a series of films. At the surface of each film interference of light takes place just as at the surface of a soap-bubble, and the more evenly the films are spaced apart the more uniform is the colour displayed, the actual tint depending upon the thickness of the films traversed by the light giving rise to the phenomenon. CHAPTER VI DOUBLE REFRACTION The optical phenomenon presented by many gem-stones is complicated by their property of splitting up a beam of light into two with, in general, differing characters. In this chapter we shall discuss the nature of double refraction, as it is termed, and methods for its detection. The phenomenon is not one that comes within the purview of everyday experience.