Diamonds And Luxury

In this section we shall consider certain optical peculiarities and colour effects of a special and more or less abnormal kind ; these features are not shown by every specimen of a particular mineral species, but only by isolated examples. These appearances are governed by the ordinary laws of reflection and  refraction of light, and are due to the the material; they consist of an unknown substance which is very feebly refracting, but it is possible, however, that some are mere vacuities. The whole of the polished surface of such a mineral may reflect the same colour, or different areas of the surface may show different colours. Except with a particular inclination of the light, however, no colour of any kind is seen. On account of this beautiful exhibition of colour, Labrador felspar is more often used for ornamental purposes than is the felspar of Fredriksvarn, the colour of which is less brilliant and variable.
On certain faces of the minerals hypersthene, bronzite, and diallage, when viewed in a particular direction in reflected light, is to be seen a metallic sheen, uniform in character over the whole surface. These minerals consist of non-metallic substances, and the metallic lustre seen on certain faces is due to the presence of numerous minute plates embedded in the substance of the mineral, parallel to certain directions. When cut and polished with a plane or curved surface parallel to these directions, these minerals are sometimes used as ornamental stones. Hypersthene shows a fine, dark, copper-red reflection ; while in the other minerals grey, yellow, green, and brown colours are predominant.

A red metallic glittering sheen is also exhibited by avanturine-quartz; here, however, it is not distributed uniformly over the whole surface, but occurs in numerous small isolated points. These can be seen with the naked eye to be due to small scales of mica enclosed in the quartz; in the same way, avanturine-felspar, or sun-stone, encloses small plates or scales of haematite.

Beautiful effects due to the modification of light are sometimes seen in minerals which possess a more or less pronounced fibrous structure. Such stones when cut with rounded surfaces in the direction of the fibres exhibit a wave of milky light travelling over the surface of the stone as it is moved about. Ordinary cat's-eye, also known as quartz-cat's-eye (Plate XVIII., Figs. 4a and 4b), consists of quartz enclosing numerous fibres of asbestos all arranged in the same direction ; the asbestos may sometimes have been weathered out, in which case the quartz will be penetrated by numerous fine hollow canals. These fibres or canals cause much the same appearance as that seen in adularia and cymophane (oriental cafs-eye), and in this case it also is known as opalescence or chatoyancy. Quartz-cat's-eye may be green, brown, or yellow, and is similar in appearance to the true or oriental cat's-eye. This similarity does not extend to the structure which is the cause of this appearance, for the sheen of quartz-cafs-eye is in reality of the nature of a fine silky lustre, such as is often shown by minerals possessing a fibrous structure, the character of which is modified, however, in the present case, by the nature of the quartz itself. Another variety of fibrous quartz is tiger-eye which is often used for cheap jewellery ; it shows a fine golden reflection, and has a marked tendency to metallic lusti-e (Plate XVIII., Fig. 5).

The appearance known as asterism belongs to the same class of phenomena; it is most frequently seen in ruby and sapphire among precious stones, but is not confined to these. When one of the hexagonal crystals of ruby or sapphire (Fig. 53, e-i) has a plane or curved surface cut at the ends, a six-rayed star may be seen by viewing a flame through the stone, or by observing the milky reflection from the surface of the stone. Such stones are known as star-, or asteriated-sapphires or rubies as the case may be, or simply as star-stones or asterias. The effect is produced by reflection of light from a multitude of extremely fine, hollow and long canals. These canals lie in one plane, and are arranged in three directions inclined to one another at 120°. The planes which contain the canals are perpendicular to the principal crystallographic (vertical) axis of the crystal, that is, they are parallel to the plane in which the stone must be cut. According to another view, the star is due to the reflection from numerous twin-lamellae arranged in three sets.

Finally, the phenomena of fluorescence and phosphorescence must be briefly described, though they are of little importance in the case of minerals used as precious stones.

Fluorescence is shown to a marked degree by fluor-spar from the lead mines of Cumberland ; the phenomenon, indeed, takes its name from this mineral, which is known to mineralogists as fluor or fluorite. A fluorescent substance appears of one colour in transmitted light and of quite another in reflected light; thus fluor-spar from Cumberland is green in transmitted light and purple in reflected light. This mineral is, however, very little used as a precious stone ; one which is used more frequently for this purpose is amber, and specimens from certain localities, namely from Sicily and Burma, show a remarkable fluorescence. Amber from these localities varies in colour from yellow to brown in transmitted light, and from green to blue in reflected light.

The rounded polished surface of such specimens shows a peculiar sheen, which, according to present tastes, diminishes the beauty of the stone, and consequently its value. Substances in which the phenomenon of phosphorescence is seen emit, when submitted to certain external influences, a soft, white or coloured light which is often only distinctly visible in a dark room. In some cases the emission of light persists for some time, while in others it lasts for a much shorter time, perhaps for only a few moments. Phosphorescence is exhibited by several precious stones, and may therefore aid in their recognition or discrimination. Two pieces of rock-crystal (quartz) phosphoresce when rubbed one against another ; diamond shows a marked phosphorescence when rubbed on cloth ; even when lightly rubbed on a coat-sleeve it will be seen in the dark to phosphoresce brilliantly. Many diamonds also phosphoresce after being exposed to the direct rays of the sun ; they store up the sunlight, as it were, in order to give it out again when placed in darkness. Lapis-lazuli from Chili phosphoresces when warmed to a temperature considerably less than that of red-heat; white topaz, some diamonds, and other minerals behave in the same way. In many minerals the phosphorescence induced by warming lasts only for a short time, but may be produced again and again on reheating.