A complete account of precious stones ucla shows the completely developed crystal after being isolated from the mother-rock.
Many precious stones and minerals, however, are found not completely embedded in the rock-mass but attached to the walls of cavities in the rock and projecting freely into the interior space. These cavities may be either completely enclosed by the rock, in which case they are of various shapes and sizes, or they may partake more of the nature of cracks and fissures penetrating the rock and varying in width and length between wide limits. The formation of minerals found inside such cavities is always of later date than that of the rock-mass itself. Such later-formed minerals may completely fill a cavity or fissure, or they may form a more or less thick incrustation on its walls.
Such cavities lined with crystals are known as drusy cavities or druses. Crystals detached from a drusy cavity will show a broken surface at the end by which they were attached to the wall of the cavity, but in other directions they will be perfectly developed in accordance with the type of symmetry peculiar to them. These attached crystals differ in this respect from the embedded crystals, mentioned above, which latter are equally developed on all sides.
The quartz crystals shown in Figs. 85 b — d, are examples of attached crystals broken away from their underlying matrix, while Fig. 85 a is a representation of an embedded quartz crystal, equally developed on all sides. In Figs. 85 b — d, the irregularly broken point of attachment of each crystal is directed downwards and is fairly large; it is sometimes, however, quite small and may be hardly observable. A group of crystals, of the variety of quartz known as 2 ct diamond earrings rock-crystal, such as frequently occurs in crevices and fissures in the gneiss of the Alps, is shown in Plate XVII.
More important than the occurrence of precious stones in primary rocks is their presence in loose, secondary deposits, which have been derived from the weathering and breaking down of primary rocks, and are known as gem-sands or gem-gravels.
The mother-rock, in which the precious stones were originally formed, has been exposed to the action of atmospheric agencies, rain, frost, &c, and has become weathered at the surface. Some of the constituents of the rock are dissolved in water and carried away and thus the cohesion of the mass is destroyed. The more or less loose, clayey, or sandy residue is the weathered product, and this will contain the precious stones which were present in the original rock, since, as a rule, they are unattacked by weathering agencies. The precious stones will be present in the weathered product in relatively greater numbers than in the original mother-rock.
It will be readily understood that it is more profitable to work weathered material than the unaltered primary rocks for precious stones, for not only is the former relatively richer in gem-stones than the latter, but it allows of the stones being easily separated or washed out. The extraction of a gem-stone from solid rock involves much labour and patience, and, even when every care is taken, may result in serious damage to the stone.
The loose, incoherent material which results from the weathering of a rock, when it contains a mineral worth extracting for technical purposes, is known generally as a sand, and as a gem-sand when it contains precious stones. It is in such sands which, wherever they occur, cover the solid rocks and form the outer portion of the earth's crust, that the most valuable precious stones are found, such, for instance, as diamond, ruby, and sapphire. They are separated from these masses of detritus by the process of gem-washing, in which the heavier stones and larger fragments remain behind, while the lighter clayey and sandy con¬stituents are washed away.
When the weathered material has not been carried away by the various transporting agencies, but remains near the parent rock, the precious stones and other minerals it contains will preserve intact the sharp edges and the crystalline form they possessed when embedded in the solid rock. Such cases, however, are rare ; much more frequently the whole of the loose material is transported in streams and rivers, and is finally deposited in a lower part of the valley, far away from its original resting-place. In such river sands and gravels, which are known as alluvial deposits, the mineral fragments, and even the precious stones, in spite of their hardness, become so rubbed by mutual friction during their travels that all angularities are lost, and they present the appearance of smooth, rounded pebbles or grains.
The presence or absence of this smooth water-worn appearance in the mineral fragments of rock detritus is conclusive proof in the one case that water has been the transporting agency, and in the other that it has not. The greater the hardness of the precious stone transported in gravels by water, the less will be the rounding it undergoes; even diamond, the hardest of materials, may show traces of rounding if the action of other softer stones is only continued long enough.
The precious stones, found in such water-worn materials, are frequently superior to specimens which have not been subjected to the action of running water, and are still to be found in their parent-rock. Such stones are frequently traversed by fissures, often scarcely visible, but enough to make them unfit for use as gems, since, as has been mentioned before, they have a tendency to break along these fissures. Precious stones which have been rolled about and ground together in the bed of a river during long ages have undergone a fairly severe trial; any which have a tendency to fragment will be reduced to splinters at an early stage of their journey ; those, on the other hand, which survive may be considered to have proved their durability.
As regards the geographical distribution of precious stones, it may be mentioned that in former times the most valuable came from India and other parts of the " Orient." It was therefore believed in the Middle Ages that the glowing sun of tropical countries was essential to the development of those qualities in precious stones which are so highly prized, and that specimens from colder countries were deficient in these qualities. Every good stone, of which the locality was not certainly known, was for this reason assumed to have come from the " Orient." A remnant of this belief still lingers in the application of the terms " oriental" to the more valuable, and " occidental" to less valuable stones. It has now long been known that the habitat of the finest of precious stones is by no means confined to the " Orient" and hot countries, such as India, Ceylon, Burma, Siam, Brazil, Colombia, &c, but that equally fine stones may also be found in North America, the Urals, and other Northern Countries. The terms" oriental" and "occidental," as now applied, have no longer a geographical signification, but refer simply to the quality of the stones to which they may be applied. Thus to distinguish cymophane from the more common quartz-cat's-eye it is termed " oriental cat's-eye"; in the same way yellow sapphire is known as " oriental topaz," and yellow quartz as " occidental topaz." The various localities in which precious stones are and have been found will be considered in detail, along with the special description of each precious stone.
The exact nature of these conditions, and the manner in which they cause the abnormal appearances, is not completely known in every case ; as full an explanation as is possible will be given, in the description of each particular case.
The play of prismatic colours exhibited by the diamond has already been dealt with in detail; we have here the simplest case of the refraction and dispersion of light.
The prismatic colours produced by cracks in the interior of a transparent stone, and best shown in colourless examples, gives rise to the appearance usually known as iridescence. The irregular fissures, or more frequently the plane cleavage cracks, inside a crystal represent narrow crevices, which may be vacuous or filled with air; these films give rise to the brilliant prismatic colours known as Newton's rings, or as the colours of thin films or plates. These colours, which are shown to perfection by soap-bubbles, are inde¬pendent of the colour of the substance itself, or of any colouring-matter contained in it, but are due to purely physical causes connected with the passage of white light through the film. The phenomenon which thus gives rise to the appearance of prismatic colours is known to physicists as the interference of light.
Some iridescent stones, such as rock-crystal, are occasionally cut so as to bring the cfackpEo which the display of prismatic colours is due, near the surface, and thus render them the more striking. Especially beautiful prismatic colours are shown by some kinds of colourless opal, namely, the so-called noble or precious opal (Plate XVI., Figs. 6-9). These colours are not shown over the whole surface of the stone, but in small discontinuous patches, closely aggregated. This appearance in opal is certainly a kind of iridescence, but as to its exact cause there is still a difference of opinion.
The translucent or semi-transparent variety of potash-felspar or orthoclase, known as adularia, sometimes shows a bluish or milky reflection of light, not from the whole surface but only from certain crystallographic planes. This opalescent appearance is specially prominent when the stone is cut and polished with a rounded convex surface, over which, when the gem is moved, a streak or wave of such reflected light passes. This appearance, being specially pronounced in adularia, is sometimes known as adularescence. It has been compared to the soft light of the moon, and specimens showing it to perfection are called moon-stones (Plate XVI., Figs. 4 and 5), and are often used for ornamental purposes. The pearly opalescence of adularia is due to reflection of light from internal platy fractures or planes of separation, and from microscopically small crystal plates embedded in the adularia along these planes.
A similar appearance is shown by some specimens of chrysoberyl, which are also valued as precious stones under the name of cymophane or cat's-eye (Plate XII., Fig. 11), since the sheen of this green, yellowish-green, or brown stone recalls the appearance of the eye of the cat. We shall see later on that there is a variety of quartz having this same appearance; the chrysoberyl variety is therefore distinguished as true or " oriental cat's-eye."
The brilliant colours shown on certain faces of labradorite, a felspar from Labrador (Plate XVI., Fig. 2), as well as by a potash-felspar from Fredriksvarn in southern Norway (Plate XVI., Fig. 3), is also, like adularescence, due to the presence of numerous minute crystal plates enclosed in the felspar and arranged parallel to these planes. The appearance resulting from the peculiar structure of these minerals is known as change of colours or labradorescence. In most positions these minerals are dull grey and unattractive looking, but certain faces in reflected light, and at a certain inclination to the light, show the most brilliant shades of green, blue, violet, red, yellow, &c. The small plates, which give rise to the reflection of coloured light, may be seen under the microscope embedded in