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.
So long ago as 1669 a Danish physician, by name Bartholinus, noticed that a plate of the transparent mineral which at that time had recently been brought over from Iceland, and was therefore called “Iceland-spar,” possessed the remarkable property of giving a double image of objects close to it when viewed through it. Subsequent investigation has shown that much crystallized matter is doubly refractive, but in calcite—to use the scientific name for the species which includes Iceland-spar—alone among common minerals is the phenomenon so conspicuous as to be obvious to the unaided eye. The apparent separation of the pair of images given by a plate cut or cleaved in any direction depends upon its thickness. The large mass, upwards of two feet (60 cm.) in thickness, which is exhibited at the far end of the Mineral Gallery of the British Museum (Natural History), displays the separation to a degree that is probably unique.
Fig. 24.—Apparent doubling of the Edges of a Peridot when viewed through the Table-Facet.
Although none of the gem-stones can emulate calcite in this character, yet the double refraction of certain of them is large enough to be detected without much difficulty. In the case of faceted stones the opposite edges should be viewed through the table-facet, and any signs of doubling noted. The double refraction of sphene is so large, viz. 0·08, that the doubling of the edges is evident to the unaided eye. In peridot (Fig. 24), zircon (b), and epidote the apparent separation of the edges is easily discerned with the assistance of an ordinary lens. A keen eye can detect the phenomenon even in the case of such substances as quartz with small double refraction. It must, however, be remembered that in all such stones the refraction is single in certain directions, and the amount of double refraction varies therefore with the direction from nil to the maximum possessed by the stone. Experiment with a plate of Iceland-spar shows that the rays transmitted by it have properties differing from those of ordinary light. On superposing a second plate we notice that there are now two pairs of images, which are in general no longer of equal brightness, as was the case before. If the second plate be rotated with respect to the first, two images, one of each pair, disappear, and then the other two, the plate having turned through a right angle between the two positions of extinction; midway between these positions the images are all equally bright. This variation of intensity implies that each of the rays emerging from the first plate has acquired a one-sided character, or, as it is usually expressed, has become plane-polarized, or, shortly, polarized.