About Quartz Crystals
Quartz, sometimes called crystal or rock crystal, is the trigonal, hexagonally structured form of crystallized silicon dioxide (SiO2). The most common form of SiO2 is low-temperature quartz. Because quartz is quite hard, (a 7 on the Mohs scale), it demonstrates a resistance to mechanical abrasion that allows it to survive many of the processes on Earth's surface. This makes quartz one of the most common minerals on Earth's surface.
Features
Quartz, similar to other minerals, forms in cavities and cracks in other rocks. Dissolved silicon dioxide is transported by hot water either as a liquid or gas (steam) into these cavities and openings and as the water cools, the mineral crystallizes out of the water. This is similar to the evaporation process of seawater leaving behind salt crystals when the water evaporates. Quartz forms primarily in igneous rocks although it can also occur in sedimentary and metamorphic rocks.
Formation
Large cavities or cracks can produce enormous quartz crystals or clusters of crystals. Quartz forms after other silicates have joined with most available cations. This process of mineral incorporation (such as can be seen on a polished granite countertop) bonds the minerals together into very strong composite materials. As the minerals bond together, the remaining cavities or growth opportunities collect the escaping water, making the quartz growth environment wet. This happens under incredible geologic pressures over significant periods of time.
How Quartz Got its Name
According to the College of Natural Resources of the University of California at Berkeley, "The name quartz is believed to have originated in the early 1500s from the Saxon word querklufterz (cross-vein ore), which was corrupted to quererz and then to quartz." suggests the word comes from Middle High German word twarc, meaning hard.
Colored Quartz
Colored forms of quartz include amethyst, citrine, rose, milky, black, smoky, rutilated, tourmaline and chatoyant intruded, ametrine and adventurine. Natural color occurs due to impurities when quartz is crystallizing. Amethyst comes in shades of purple, due to radiation damage and the presence of iron. It is often used as a gemstone. Amethyst received its name from the ancient Greek word methys, meaning not drunk. It was believed that the stone prevented drunkenness, according to Etymology Online. Citrine comes in shades of orange and yellow orange and it is also often used as a gemstone. It is also colored by the presence of iron. When amethyst and citrine form together they create ametrine and produce a bicolored peach stone.
Types
Smoky quartz can range all the way into black. It is colored by aluminum and radiation. Etymology Online notes that this stone is also called a touchstone. It was used in ancient times to test for the quality of gold and silver alloys. Rose quartz is pink due to the presence of titanium and it is frequently used in bead work. Green quartz (Prasiolite) is rare and comes from a mine in Brazil. Amethyst is sometimes heat treated to make it change to a green color, but it isn't the same as the real stone. Milky quartz is often associated with deposits of gold. Rutilated quartz is clear fine (venus hair) red rutile crystals. Tourmaline and chatoyant intruded quartz are clear quartz with intrusions of tourmaline or asbestos. Adventurine is green quartz with intrusions of mica. There can be other intrusions that are more rare. Other interesting properties of quartz include its piezoelectric qualities and, if you rub two quartz crystals together, they produce light, a feature called triboluminescence. And, although quartz is extremely hard, it can be dissolved in very hot water.
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