Photo of Rock Crystal (Quartz) in Processed & Rough Form
Rock Crystal (Quartz) is a mineral with a hardness of 7 out of 10 on the Mohs scale of mineral hardness [?]. These Trigonally structured gems are made of silicon dioxide, their full chemical compound being SiO2.
Rock Crystal is the purest water-clear and colorless from of Quartz. It is known as mountain crystal (Bergkristall). It is the presence of impurities that gives other varieties of quartz their colors.
It is found in beautifully formed crystals, often with complex terminations. These are usully bounded by the faces of six-sided prisms, which are almost always striated horizontally. Very often they are doubly terminated.
It is found in crystals of microscopic dimensions as well as in crystals as large as 5 metric tons. Crystals often contain bubbles of gas or liquid clearly visible to the naked eye.
One characteristic of such crystals is that there are two different rhombohedrons composing each end, or termination, resulting in six triangular faces, alternate ones being larger or smaller than its neighbor.
An asymmetrical arrangement of so-called trapezohedral faces leads to the development of left-handed and right-handed quartz crystals.
Quartz is, after the feldspars, the second most common mineral and accounts for about 12% of the earth's crust. It crystallizes directly from igneous magma, from the pegmatite-pneumatolytic to the low-temperature hydrothermal stage. It also crystallizes from hot solutions (geyserites) and cold solutions (hyalites) and occurs as a diagenetic mineral derived from skeletons of certain organisms (diatomites, chert).
As a major rock-forming component, it is found in numerous magmatic, sedimentary, and metamorphic rocks. Examples are granites, rhyolites, gneisses, sandstones, and quartzites.
In masses weighing many thousands of tons, it often forms the main core of pegmatites, or is a dominant mineral of hydrothermal vein fillings. The highly coveted rock crystal, along with brown smoky quartz and violet amethyst and other quartz varieties, are formed in cavities and vugs of massive rocks, under moderate hydrothermal conditions, the open spaces of cavities allowing the crystals to grow unhindered.
During and after weathering of rocks, quartz, because of its chemical and mechanical resistance, remains almost unchanged. It can, therefore, be transported and rounded, over thousands of years by water, ice, and wind, and ultimately deposited in broad, and frequently massive accumulations of boulders and sand. Very often when surrounding rocks are weathered away, remnants of quartz in giant blocks will be left behind.
The largest rock crystals come from Minas Gerais, Brazil. It is the most important supplier of rock crystal. Apart from Minas Gerais, important deposits may be found in the states of Goyaz and Bahia.
Famous for the specimens they have produced are the "crystal cellars" of the French and Swiss Alps. Especially prolific cleft systems occur at Dauphine, near Bourg d'Oisans, and on Mont Blanc in the Aare-Gotthard Massif, in upper Valais and Tessin. Others are found in the Zillertal Alps and the Hohen Tauern. The largest rock crystals from Alpine clefts reach a size of more than 1 meter.
Beautiful rock crystal is also found in Madagascar. Water-clear, fully developed crystals of small size are found in cavities in marble at Carrara (Italy), and in the sandstones near Herkimer in New York (USA). These are called "Herkimer diamonds."
Rounded, abraded rock crystals, most probably originatins from the Gotthard massif, once found in the Rhine River, were the original "Rhinestones." The rhinestones that are sold today are simply sparkling imitations made of glass.
Misleading terms for rock crystal quartz include Alaska diamond, Alenon diamond, Bohemian diamond, Cornish diamond, Bristol diamond, Herkimer diamond.
The specific gravity [?] for Rock Crystal (Quartz) is 2.65, its refractive index [?] is 1.54-1.55, and its double refraction [?] is 0.009.
History
Rock crystal had already been used as early as 3000 B.C. Jewelry, bowls, and cosmetic containers were found in ancient Egyptian graves.
In old legends, it was believed that nymphs and fairies lived in palaces made of glittering rock crystal deep within the mountains.
The Greeks called the transparent quartz "krystallos," meaning "the ice." Pliny had written, "In any case it is only found where the winter snow brings the most cold, therefore it must be formed from crushed ice and pure snow."
The stone was also highly valued by the Romans. Emperor Augustus consecrated on the Capitol the largest rock crystal known at the time, and Emperor Nero favored drinking from large crystal cups. According to Pliny, Roman doctors used rock crystal spheres as "burning glasses" to concentrate the sunlight for burning out wounds. With the aid of "magic spheres" of rock crystal, one could allegedly see into the future.
The view that the "krystallos" was petrified ice which could not be melted by the hottest rays of the sun was widespread in ancient times. Johannes Kunckel, chemist and discoverer of ruby glass, reported that rock crystal was nothing more than coagulated ice, in 1689 in his book Ars Vitria (The Art of Glassmaking), even though the physicist and chemist Robert Boyle effectively discredited this legend in 1672, in his work Essay on the Origin and Properties of Precious Stones. He determined that the specific gravity of "crystall" was three times that of ice and that the two, therefore, could not be identical.
For thousands of years the term "crystal" was used only to describe rock crystal quartz. Only later, after Nicholas Steno in 1669 first used quartz to discover the principal of the constancy of the angles between crystal faces, was the term expanded to include other symmetrically formed crystalline objects. The science that dealt with these properties in particular was from that time called crystallography.
Industrial Usages
Clear, untwinned rock crystals serve as oscillators to stabilize the broadcast frequencies of radio stations and wireless signals. It is used in electro-acoustics to produce ultrasonic signals or to drive electronic oscillators (quartz clocks). In all but a few localities, such as Brazil and Madagascar, most natural quartz crystals, including those of the Swiss Alps, are twinned, and are not useful as piezo-electric quartz. For this reason, huge quantities of synthetic crystals are produced.
High quality rock crystal is also used in the manufacture of optical instruments, especially as lenses and prisms. Quartz glass, formed by melting natural quartz, is very resistant to chemical agents, is not sensitive to extreme temperature changes, and is very transparent to ultraviolet light. For these reasons it is used widely in the manufacture of chemical apparatus and ultraviolet lamps.
Pure quartz sand or vein quartz are the most important raw materials in the glass and porcelain industries, and they are also important in the production of ferrosilica and carborundum, which is an excellent cutting abrasive due to its extreme hardness.
Impure quartz sand is widely used in the construction industry as an additive in cement and mortar, in the metallurgical industry as a sandblasting, and also as a filtering agent in preparing drinking water.
Rock crystal is seldom used as a faceted stone, (the cut of which would show its inferiority to diamond), but more often it is used for matte-cut bead necklaces, often in combination with beads of other very colorful minerals, such as lapis, sodalite, and chrysoprase. Rock crystal with included crystals of other minerals, such as hairlike rutile needles (called "Venus' hair") and even without inclusions, is widely used for carving figures and artistic objects. Crystal balls are ground and polished, particularly in Japan.
It has been used much more frequently for the fashioning of elaborate, finely engraved cups, jugs, plates, and vases. Masterpieces have been produced in the past and there are still places where the tradition is continued.
As a gem, its value is extremely low. As an ornamental material, its value largely depends on the way in which it is fashioned. Fine examples may be quite valuable and more costly than similar pieces made from more opaque materials. Finely worked antique pieces are of course still more valuable.
Laboratory-produced rock crystal is made only for technological purposes.
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