Rock crystal

“ This article deals with the plainly crystalline varieties. These include, among other varieties, rock crystal, amethyst, rose quartz, smoky quartz, and sagenitic quartz. These varieties all occur in well formed crystals, and all have a vitreous luster, i. e., luster like that of glass. The differences between them are almost exclusively differences of color.
Rock Crystal—This is quartz in its purest form. Typical rock crystal is perfectly transparent and colorless, but the mineral is often more or less clouded and opaque. ”

“ As rock-crystal, the typical form of quartz, is an hexagonal prism terminated at each end by a rhombohedron, when broken it will be seen to have a conchoidal or splintery fracture. Rock-crystal is very widely distributed, being found in rocks of all ages. The most beautiful and perfect specimens are usually obtained from large cavities or geodes in the older igneous rocks, and also from veins in these and other rocks. The size and color of quartz-crystals vary greatly; some are so small as to be microscopical, while others are of very considerable bulk. ”

“ Objects of Rock Crystal. Sepulchre III. Actual size.
There was further found here the perforated ball of beautifully-polished rock crystal (No. 307) , which has evidently formed part of the handle of a sceptre or some weapon, for we see in the perforation a long piece of metal which appears to be gold, but which is probably only gold-plated bronze or silver.
The object, No. 308, is also of well-polished rock crystal; it has a large mouth, and a perforation on the opposite side; the interior has a lively ornamentation painted in red and white. ”

“ You must say, jet will take up a straw;
amber will make one fat;
Coral will look pale when you be sick,
and crystal stanch blood. That a remedial tincture of rock-crystal could be made was firmly believed by the Danish chemist, Ole Borch (Olaus Borrichius, 1626–1690) , and in his chemical lectures he gives the following directions as to the processes to be employed. A rock-crystal was to be heated to a high temperature and then cast, while still warm, into cold water ”

“ Among the numerous localities in the United States which yield rock-crystal mention may be made of those in Herkimer Co., New York State, whence the Lake George crystals are obtained; and it is notable that some of the Herkimer quartz encloses bituminous matter. Mokelumne Hill, Calaveras Co., California, has furnished some remarkable rock-crystal. In Europe the localities are very numerous, the most important being those in the Alps. ”

“ I reminded him of his having spoken of crystals of ice, and asked how that term could be applied to any thing but mineral bodies.
“The term crystal,” he replied, “came from the Greek word for ice—it was afterwards applied to rock crystal, which the ancients imagined to be water converted into stone; but it now signifies the regular figure in which the particles of any substance arrange themselves in passing from the liquid to the solid state.—Each of those sub{86}stances has a figure for its crystal peculiar to itself, and from which it never varies. ”

“ The limpid and colourless kinds are often called Bristol or Irish diamonds, after the various localities in which they are found. Rock-crystal frequently occurs in the Alps, as is well known to every traveller in Switzerland. Small rock-crystals have hardly any value, but considerable prices are paid for very large specimens, which are accordingly much sought for by chamois-hunters and goatherds. About a century since a quartz cave was opened at Zinken, which afforded 1,000 cwt. of rock-crystal, and at that early period brought 300,000 dollars. One crystal weighed 800 pounds. ”

“ In the back streets it is not uncommon to find places where they make them, and others where artists are engaged in cutting and polishing on the lathe the few precious stones they possess. Rock crystal is one of their favorites, and from it they cut beautiful vases and goblets that are sometimes as clear as glass. In this, however, they are surpassed by the Japanese, whoso crystal globes cannot be distinguished from the most perfect glass. They also cut vases and carve odd figures in an arsenical stone, of reddish color, with a grain like granite, which is little known in other countries. ”

“ Articles found while removing the Stuff formerly wheeled from off the Mound, i.e. above the Upper Log-Pavement.
Fig. 261.—Outline of Crystal Ornament (1⁄1) .
(1.) Rock Crystal.—A conical piece of rock-crystal, evidently ground down to its present shape. The diameter of base is 1⁄16 less than an inch, and the perpendicular height is 3⁄4 of an inch. The base is not quite flat, but slightly convex, as will be seen from the annexed outline (Fig. 261) . It scratches glass, but is scratched by a diamond, and depolarises a ray of light. Its specific gravity is 2·64.
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(2.) ”

“ I have carefully studied crystal-gazing, and though I have remarked individual differences in each sensitive, I think I may say that, as far as working methods are concerned, I have come to the following conclusions:—
The material of which the object is composed is not a matter of indifference. Balls of rock-crystal have given me the best results. I have seen people, incapable of receiving visions with ordinary glass, obtain them in a tiny ball of natural crystal. Objects in rock-crystal have the inconvenience of being very expensive. ”

“ At other times a crystal which is imbedded in rock has been attacked upon its surface by the percolating solutions, and the dissolved materials have been deposited in place as a crown of new minerals which steadily widens its zone until the center is reached and the original crystal has been entirely transformed (Fig. 14) . It is sometimes possible to say that the action by which these changes have been brought about has involved a nice adjustment of supply of the chemical constituents necessary to the formation of the new mineral or minerals. ”

“ Imperfect crystals.—
A crystalline mineral commonly forms under conditions that do not permit it to become a well-shaped crystal. Although the mineral may show a few crystal faces, it does not have a complete crystal shape and so is described as massive, or is said to occur in masses. Some of the minerals that make up rocks occur as crystalline masses. For example, calcite is a crystalline mineral that occurs in the metamorphic rock marble without its normal crystal shape. ”

“ Plate X. Photograph of a cast made by pouring plaster over a surface of mud in which ice crystals had recently formed, in Rock Island, Ill., and where they had later been removed by natural sublimations into the atmosphere, leaving open cracks in the mud. It will be seen that the crystal growth in this case involves an x-like or radiating pattern formed of relatively very slender forms that almost everywhere are very gently curved somewhat reminding of the slender thread-like crystals sometimes seen in frost on windows. ”

“ Still more rare is a mineral, of which singularly enough we possess as yet no satisfactory analysis, known as Turnerite, from the Tavetsch Valley in the Alps. In this collection there is a number of perfect crystals, five if I remember correctly, each of which displays many highly-lustrous facets, and occurring both isolated and embedded in a rock made up of quartz and albite. The largest crystal is three-quarters of an inch long and five-eighths of an inch thick. It is a doubly-terminated hexagonal prism, the basal edges being regularly replaced by twelve small planes. ”

“ On damoselle than doth rokette.” Rock-crystal. A material much used for carving in China. Fortune says, “Fine specimens of rock-crystal, carved into figures, cups, and vases, are met with in the curiosity shops of Foo-chow-foo. Some of these specimens are white, others golden yellow, and others again blue and black. One kind looks as if human hair was thrown in and crystallized. Imitations of this stone are common in Canton, made into snuff-bottles, such as are commonly used by the Chinese.” The Greek name (κρύσταλλος, ice) refers to the belief that it was frozen water ”

“ Rock Crystal grows ordinarily in hexagonal bars, and diamonds are found which occur with a square point and polished surfaces. There is a species of small flat stones, piled up directly upon one another, which are all of pentagonal figure with rounded angles, and the sides a little folded inwards. ”

“ In all probability, in Nature many precious stones have formed in the same manner; and most mineralogists concur in the opinion that rock-crystals, consisting of nothing but silicic acid, and frequently weighing hundreds of pounds, have originated thus. It is almost certain that this formation from liquids into solid bodies has taken place in a large class of half-precious stones, such as quartz and pyrites, consisting likewise of nothing but silica—namely, agate, jasper, opal, chalcedony, chrysoprase, carnelian, heliotrope, and others. ”

“ A crystal is a mineral, bounded by symmetrically grouped faces, which have definite relationships to a set of imaginary lines called axes. There are between 1100 and 1200 minerals, of which 30 are so frequently present, and so dominant in making up the rocks, that they are termed rock-forming minerals. ”

“ Gemstones that have an orderly internal molecular arrangement are referred to as crystalline. This internal order is commonly reflected in the external shape of “rough” or uncut gemstones. The resultant shape is a polyhedral solid bounded by planes and called a crystal. Well-formed crystals are formed in nature only under relatively ideal conditions of temperature, pressure, and space. The specific temperatures and pressures involved vary with different minerals, but most crystals need space in which to form so that their “growth” is not impaired by surrounding rocks and minerals. ”

“ Inclusions of foreign matter, accidentally caught up during growth, are frequently present in crystals. Inclusions of other minerals are specially frequent and conspicuous in crystals of quartz, and crystals of calcite may contain as much as 60% of included sand. Cavities, either with rounded boundaries or with the same shape (“negative crystals”) as the surrounding crystal, are often to be seen ”

“ The shape of a crystal does not depend like that of the animal or the plant upon the life which its forefathers have led or the conditions under which they have grown.
There are indeed other features in which a mineral or any crystal may resemble a living thing in a way even more surprising than in its form. Two of the most remarkable are these: it grows out of a solution as though it were alive; and, if it is wounded or broken, it heals itself and replaces the missing part just as a living organism may do. ”

“ When a mineral occurs as a well-formed individual crystal, it has a definite, precise shape. The kind of crystal shape it has depends on its own type of crystalline internal structure. A well-formed crystal has smooth, flat, outer surfaces called crystal faces, which are arranged together to form prisms, cubes, pyramids, and many other geometric shapes. For example, quartz, a common Texas mineral, is commonly found as a six-sided, prism-shaped crystal that is topped by pyramid-like forms. Pyrite, another common mineral, occurs as cube-shaped crystals. ”

“ The crystals are clear and when pure colorless, but there is a tendency for some slight impurity to color them almost any hue.
The most perfect double-ended crystals form only where growth is possible in all directions, as in clay. In cavities and caves there is an opportunity for the crystals to grow in toward the open spaces, and in such places, one finds fine large crystals; the Alps, Brazil, Japan, and Madagascar being especially famous localities. The largest quartz crystal on record is one 25 feet in circumference which came from Madagascar. ”

“ I pointed out, I remember, that these delicate forms are, like the frost patterns on a window-pane, really expressions of the crystalline shape and symmetry of the mineral; each mineral consists of crystals, and therefore has its own peculiar crystalline shape which is one of its inherent properties; moreover, this persists unchanging through the ages, and under all conditions, and is in no way dependent upon the environment in which the mineral is situated. ”

“ Organisms mostly grow from within, while crystals grow from material externally added. It is an astonishing fact that in crystals as well as organisms growth takes most rapidly on a wound or broken place. Thus if a crystal is removed from the solution in which it is growing and put back after a corner has been broken off, the fractured surface will build up more rapidly than the rest. Finally, crystals are not necessarily limited in age like organisms. Under certain natural conditions, as, for example, weathering, crystals may decay or be broken up ”

“ Eventually, while the solution is at rest in a protected place, free from draughts or vibration, and after it has cooled to the temperature of the air, a crystal-germ enters, followed probably by others; each forms a centre of crystal growth, which proceeds very slowly and deliberately, keeping pace with the evaporation in such a manner that the labile condition is never reached. The natural result is the production of very well-formed crystals bounded by excellent faces, truly plane and free from striation or distortion. ”

“ Since there are about 1,000 mineral species and only 232 fundamental forms, it necessarily follows that two or more species may crystallize in the same form within a class, so that it is not always possible to tell the species of mineral merely by its crystal form. It is, however, a remarkable fact that, where two or more substances crystallize in the same class (i.e., show the same grade of symmetry) each substance almost invariably exhibits “crystal habit” which is a pronounced tendency to crystallize in certain relatively few forms or combinations of forms out of the many possibilities. ”