Quartz crystal

“ Opaque dark green, brown, or black Picotite or chrome spinel: Translucent dark yellow-brown or green-brown QUARTZ
(INCLUDES ROCK CRYSTAL, AMETHYST, AND CITRINE)
Few gemstones can compete with quartz for variety of color. Having a hardness of 7 and occurring in many beautiful varieties, only the relative abundance of quartz prevents the species from attaining top rank among gemstones.
The two kinds of quartz, crystalline and cryptocrystalline (fine-grained) quartz, occur in all kinds of mineral deposits throughout the world. Much of this material is suitable for cutting gems. ”

“ Then placing one arm of the bent wire along the base of a protractor, the point of flexure at the center of the base, the number of degrees between the two arms may be read, thus giving roughly the angle between the prism and pyramid faces of the quartz crystal. The great hardness of quartz is apparent in that it cannot be scratched with the point of a knife and that it will cut glass. Often clear parts of quartz crystals occur studding the surface of a rock structure, in the form known as a crystal aggregate. ”

“ Or it may grow chiefly at two or three especially favourable places, and in the course of a few weeks, under suitable conditions, at each place a perfect little quartz crystal will radiate out from the sand grain, composed of a miniature hexagonal prism terminated by the well-known pyramid, really consisting of a pair of 3trigonal (rhombohedral) pyramids more or less equally developed, and together producing an apparently hexagonal one. Four such grains of sand, from which quartz crystals are growing, are shown in Fig. 1, as they appear under a microscope magnifying about fifty diameters. ”

“ Quartz is composed of the two elements occurring the most abundantly in the earth's crust, silicon and oxygen, both non-metals. As already indicated, the most common representative of the mineral substance is the sand of the soil. The sand grains are generally so eroded by the atmosphere and surface waters as to show little of the true quartz structure. A typical specimen of quartz, commonly known as "rock crystal," clear, transparent and approximately perfect in form, is not difficult to obtain for study. ”

“ Here is nobly born quartz living with a green mineral, called epidote; and they are immense friends. Now, you see, a comparatively large and strong quartz-crystal, and a very weak and slender little one of epidote, have begun to grow, close by each other, and sloping unluckily towards each other, so that at last they meet. They cannot go on growing together; the quartz crystal is five times as thick, and more than twenty times as strong [Footnote: Quartz is not much harder than epidote; the strength is only supposed to be in some proportion to the squares of the diameters.] ”

“ 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. ”

“ Almost every crystal of quartz found in such granite rocks displays thousands of small cavities filled with liquid and a bubble, although it is very rare to find one with so good a cube of salt and having the configuration of a quartz crystal for the shape of the cavity. ”

“ It is abundant as a colorless, transparent substance; it also appears as a brilliant crystal. The name quartz is from the German zwerg, meaning dwarf. Similarly cobalt and nickel are from German words for sprites, the gnomes being little creatures that work the mine of the gods.
In its crystalline form quartz includes amethyst, cairngorm, citrine, quartz cat’s-eye, rock crystal, and rose quartz. Another main group in the quartz family is chalcedony, which includes agate, bloodstone, carnelian, jasper, moss agate, onyx, sard, and sardonyx. ”

“ Crystals of quartz varieties that are opaque or that contain visible inclusions normally are cut as cabochons to take advantage of the body color or to make the inclusions more visible. Crystals of the transparent varieties are fashioned in any of several cutting styles, depending on whether it is desired to take maximum advantage of color or of brilliance. Because of its availability in fairly large, flawless pieces in various colors, quartz has been used extensively in carving. The Chinese have excelled in carving large, ornate objects of rock crystal. ”

“ 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. ”

“ Those who cannot afford quartz labrets substitute wood, glass, or pewter. M. Lacroix draws our attention to the fact that a study of the processes employed in shaping and polishing these pieces of quartz is of great importance for the elucidation of the methods in use during the Stone Age. [700]
A nose-jewel from the New Hebrides consists of a crystal of hyaline quartz reduced to a cylindrical form, one extremity having been pointed, while the other retains the natural faces of the crystal. ”

“ Occasionally a natural biquartz is obtained, on cutting a plate out of a crystal of quartz perpendicularly to the axis. For it is not uncommon to find a crystal which, while apparently a single crystal, is really a twin, the two right and left individuals being joined by an invisible plane of contact, or “plane of composition” as it is called, so beautifully have the two grown together. ”

“ The prismatic portion of the assemblage thus formed shows (see Fig. 15) , on its alternate edges, faces of54 molecules with projections and faces of molecules with grooves; and shows only orientational differences between alternate faces, whether of the pyramid or of the prism. Having gone only so far from ‘senary’ symmetry, we have exactly the triple, or three-pair, anti-symmetry required for the piezo-electricity of quartz investigated so admirably by the brothers Curie18, who found that a thin plate of quartz crystal Fig. 16. Fig. 17. ”

“ No reliance can be placed upon a solitary admeasurement taken upon the natural planes of one crystal, however brilliant and perfect they may appear; it is essential that several should be found to exhibit a perfect agreement, before it can be assumed that the actual value of any angle has been obtained. From one crystal of quartz, nearly perfect at both terminations and affording pretty clear reductions, I obtained six admeasurements of the same angle from its various planes. ”

“ Hence, there can be no doubt whatever that the remarkable optical behaviour of quartz is due to its point-system being of a helical nature, a right or a left-handed screw structure being apparently produced in nature with equal facility. The circumstances of environment during the formation of the crystal probably determine which variety shall be produced, and when 200the nature of the environment becomes changed during the operation of formation either twins are produced of the two varieties, or separate individual crystals. ”

“ Among the most varied and beautiful forms of quartz which have had a purely aqueous origin are all the varieties of crystalline and amorphous silica, which frequently coat the interiors of geodes and other hollow spaces in the igneous rocks, and which consist chiefly of an intermingling of chalcedony and jasper, and are conveniently grouped under the general name of agates. Pure rock-crystal, amethyst, cairngorm, and other valuable crystallized forms of quartz, are often found in connection with the same rocks, or in others of a more purely metamorphic character. ”

“ When two rhombohedra are equally developed, one being rotated with respect to the other 60° round the vertical trigonal axis of symmetry, they together 172resemble a hexagonal pyramid, and crystals of quartz thus terminated at both ends are not uncommon, so that at first sight a quartz crystal might be mistaken for a hexagonal prism doubly terminated by the hexagonal pyramid, and the mineral considered, in error, to belong to the hexagonal system. ”

“ In these, the crystals have not only been fractured in the same way, across their axes, but the fractures are filled by the quartz and felspar which constitute the body of the rock. The granite veins of Arran do not show more clearly the ramification of a central substance through the fractures of the neighbouring rock, than these specimens show the veins of quartz proceeding from the mass, and penetrating every fissure which had been formed in the crystal. ”

“ Quartz.—Oxide of silicon or silica, SiO2. Both an important vein mineral associated with the ores and a rock maker. In the former case particularly, often in crystals of notably simple forms (Fig. 487, 7) . Few minerals which are not gems are so hard. Remarkable freedom from cleavage so that the mineral breaks much like window glass—conchoidal fracture. Wide range in both transparency and color. Transparent and colorless crystalline variety (rock crystal) , brown translucent (smoky quartz) , turbid white (milky quartz) , and various colored varieties (carnelian, jasper, jet, etc.) ”

“ When a plate of either quartz or calcite one-fiftieth of an inch thick, cut perpendicularly to the optic axis, is examined under the polariscope or polarising microscope, the dark field is unaffected by its introduction on the stage, remaining dark on a complete rotation of the crystal plate in its own plane. Moreover, the calcite plate continues to behave similarly however much the thickness is increased, the field remaining dark. But when quartz is examined as regards the effect of thickness an extraordinary thing happens. ”

“ Quartz is composed of silica (SiO2) , and sands such as the ancient Illinois sand that are composed of quartz are known as silica sands. Quartz is very hard and will scratch glass and some steel. Perfect quartz crystals, which are rare, are longer than they are thick and end in pyramids. Probably not many grains of the ancient sand came from perfect crystals; they more likely resulted from the decaying and breaking down of rocks such as granite, which are mixtures of quartz grains and other mineral particles. ”

“ When molten masses of igneous rock were cooling the quartz crystals had their faces interfered with as they grew, and we have resulting crystalline quartz, simply filling in the spaces between the other crystals, such as feldspar and mica, in the granite. Quartz is a large component in many igneous rocks, also in metamorphic rocks, and certain sedimentary rocks like sandstone are almost wholly made up of quartz grains. Quartz is also the gangue mineral in many veins. ”

“ Some by their refulgent splendor rival the colors of the painters, others the flame of burning sulphur or of fire quickened by oil.”213 Possibly some brilliant varieties of iridescent quartz—“iris” quartz, possessing an internal fracture, displays with great brilliancy all the colors of the rainbow, sparkling with wonderful clearness in its field of transparent mineral—might excite the admiration of one who had never seen an opal. Referring again to these quartz crystals, they are often cut so as to form a dome of quartz and are even used as distinct jewels. ”

“ The fairest cubic crystals are slightly attached to the points of the quartz crystals. Yellowish crystals of carbonate of lime are often interspersed among the lead. Sulphate of barytes occurs here also; sometimes in plates meeting at various angles, and forming chambers lined with minute crystals of quartz. Minute crystals of copper pyrites and a little fluate of lime have been found here; also fine specimens of bitter spar. The walls are very compact, fine-grained granite.
760 feet. Coarse, parti-coloured granite. The felspar is flesh-coloured and white; the quartz often bluish or greenish ”

“ It is a very hard rock which rings under the hammer and weathers into rusty brown blocks. The sp. gr. is 2·71. Under the microscope the quartz crystals are seen to be much cracked. There are also porphyritic crystals of orthoclase and oligoclase, mostly of irregular shape, and a few crystals, of still more irregular form, of dark green hornblende. The cryptocrystalline ground mass consists chiefly of quartz and felspar, through which are scattered tiny granules of dark green hornblende. The ground mass shows a matted texture between crossed nicols; there is no trace of fluidal movement. ”

“ Many ancient rhyolitic quartz-porphyries show on their weathered surfaces numerous globular projections. They may be several inches in diameter, and vary from this size down to a minute fraction of an inch. When struck with a hammer they may detach readily from the matrix as if their margins were defined by a fissure. If they are broken across their inner portions are often seen to be filled with secondary quartz, chalcedony or agate: some of them have a central cavity, often with deposits of quartz crystals ”