Description
Rock crystal is the colorless and most common variety of the mineral quartz (silicon dioxide, SiO2). The name "rock crystal" is derived from the Greek word "krustallos," meaning ice. It was believed that rock crystal was ice shaped by the gods. Rock crystal was already known during the Mesolithic period. During the Roman period, it was brought from the Alps under the name "petrified ice." Theofratus called it "krystallos." Pliny the Elder also mentions it. The name "quartz" was introduced in 1529 by Georgius Agricola. Eastern peoples considered rock crystal the stone of patience and perfection. Tibetans use rock crystal to treat wounds. The choice of rock crystal as a drinking glass or as part of medieval jewelry can be attributed to the belief that these crystals would break or discolor when they came into contact with poison. Thus, the chain became an amulet that protected the king. Compare the Rudolphinian imperial crown and the chain of the Jehovah's Witnesses. Origin: Megnathites, pegmatites, hydrothermal and alpine veins, alluvial deposits. Occurrence: Rock crystal is widespread in nature, but not always in gem quality. Historically known sources are India and Sri Lanka (the Tatnaputi area). Iridescent quartz from the Poona region in India and Myanmar is rare. Beautiful crystals come from Kenya and Madagascar, as well as Brazil (Minas Gerais). Goisas, Bahia, and French Guiana. In the United States, rock crystal occurs in pegmatites, including in Maine (near Ausburn), New York, North Carolina, Arkansas (Crystal Peak), and California. Of particular significance are occurrences in the Alps. Rock crystals are found in Switzerland (for example, near Urim, where a 135 kg crystal was found in the area of St. Gotthard and Grimsel), and in Austria (in 1965, a crystal weighing almost 1000 kg was found in a cavity near the Grossglockner). Rock crystals are also found in Germany, the Czech Republic, Poland, and France. Unique cut rock crystals are held in the collections of various museums. The Smithsonian Institution, Washington, owns a 7000-carat cut stone and a spherical rock crystal with a diameter of 33 cm and a weight of 48.5 kg; this one originates from Burma and was cut in China.
Silicon carbide (SiC), also known as carborundum, is an extremely hard material widely used for grinding and polishing, in the form of grinding wheels, polishing and grinding powders, and as a sandpaper coating, and nowadays also for armor plating on armored vehicles. The compound was discovered by Edward Goodrich Acheson, who obtained a patent for it on February 28, 1893. Granular silicon carbide is produced commercially through a very high-temperature reaction. High-quality silica sand and carbon (usually petroleum coke) are mixed in the correct proportions to the reaction material. A pile of material is built from this mixture. At the center of this pile of material is a graphite core that serves as an electrical conductor. The graphite core is connected to a transformer and energized. Due to the resistance, the temperature in the core of the pile of material (furnace) rises to approximately 2700 degrees Celsius. At this temperature, the reaction SiO2 (sand) + 3C (coke) yields SiC + 2CO (carbon monoxide); in short: SiO2 + 3C → SiC + 2CO A clear distinction must be made between granular silicon carbide and fibrous silicon carbide, which is produced on a nanoscale for specific purposes. Granular silicon carbide has been shown to be toxically inert. Granular silicon carbide, as produced in the Netherlands, is not classified as a hazardous substance or preparation according to EC Directive 67/548/EEC or Directive 1999/45/EC or Regulation (EC) No 1272/2008. In a report published on December 7, 2012, the Dutch Health Council stated that silicon carbide in fiber form (fibers, "whiskers") can cause cancer and should be classified as "carcinogenic to humans" (category 1A), according to the Health Council's classification system. Data on the granular form of silicon carbide are insufficient to classify its carcinogenic properties (category 3). The reason is that the committee is concerned about whether commercial granular silicon carbide is sufficiently free of fiber-form silicon carbide
