common chameleon
If you gaze deep into a crystal ball, you will see a versatile gemstone, one of the most popular gems on earth. Beautiful quartz, the 'rock crystal' used in ancient times to make crystal balls and bowls, is today more often seen set in gold jewellery. Despite the popularity of quartz gems like amethyst, citrine, ametrine, rose quartz, onyx, agates, chrysoprase, rutilated quartz and other varieties, many people in the jewellery industry take quartz for granted because of its affordable price.
Throughout history, quartz has been the common chameleon of gemstones, standing in for more expensive gemstones ranging from diamond to jade. But the incredible variety of quartz is now beginning to be appreciated in its own right.
Rose quartz
The pale pink colour of quartz, which can range from transparent to translucent, is known as rose quartz. The colour is a very pale and delicate powder pink. Transparent rose quartz is very rare, and usually so pale that it does not show very much colour at all except in large sizes. Translucent rose quartz is much more readily available, being used for beads, cabochons, carvings, and architectural purposes.
Smoky quartz
Smoky quartz is a brown transparent quartz that is sometimes used for unusual faceted cuts. The commercial market is limited, because there is a rather limited demand for brown gemstones. This variety was sometimes known as smoky topaz in the past, though the term is incorrect and misleading.
Tiger's eye
Tiger's eye quartz contains brown iron which produces its golden yellow colour. Cabochon cut stones of this variety show the chatoyancy (small ray of light on the surface) that resembles the feline eye of a tiger. The most important deposit is in South Africa, though tiger's eye is also found in Western Australia, Burma (Myanmar), India and California.
Rock crystal
The transparent, colourless variety of quartz is still known as rock crystal. Long ago, people believed that rock crystal was a compact form of ice: in fact 'crystallos' means 'ice'. The best rock crystal has the clarity and shimmer of water. Although colourless quartz is relatively common, large flawless specimens are not, which is why crystal balls these days are made of glass, not quartz. Rock crystal has often been used in jewellery, particularly carved pieces. Many stunning art deco jewellery designs featured the black and white quartz combination of rock crystal and onyx. Colourless quartz crystals have also become popular in jewellery due to the popularity of legends about their powers. Many people believe that wearing quartz crystals benefits their health and spiritual well-being.
Rutilated quartz and tourmalinated quartz
While most varieties of transparent quartz are valued most when they show no inclusions, some are valued chiefly because of them! The most popular of these is known as rutilated quartz. Rutilated quartz is transparent rock crystal with golden needles of rutile arrayed in patterns inside it. Each pattern is different and some are breathtakingly beautiful. The inclusions are sometimes called Venus hair. Less well known is a variety called tourmalinated quartz which, instead of golden rutile, has black or dark green tourmaline crystals.
Chalcedonies
Quartz that is formed not of one single crystal but a number of finely grained microcrystals is known as chalcedony. The variety of chalcedonies is even greater than that of transparent quartz, including cryptocrystalline quartz with patterns as well as a wide range of solid colours. Agates are banded. Bloodstone has red spots on a green background. Moss agate has a plant-like pattern. Jasper sometimes looks like a landscape painting. Another staple of the jewellery industry is black onyx, chalcedony quartz which owes its even black colour to an ancient dyeing process that is still used today. Carnelian, another chalcedony valued in the ancient world, has a vivid brownish orange colour and clear translucency that makes it popular for signet rings and seals. Chrysoprase, a bright, apple-green, translucent chalcedony, is the most valued. It was a particular favourite of Frederick The Great of Prussia. It can be seen today decorating many buildings in beautiful Prague, including the Chapel of St Wenceslas. Today, chrysoprase is found mostly in Australia. Unlike most other green stones, which owe their colour to chromium or vanadium, chrysoprase derives its colour from nickel. Its bright even colour and texture lend themselves well to beads, cabochons, and carvings.
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Quartz is the second most abundant mineral in the Earth's continental crust, after feldspar. It is made up of a continuous framework of SiO4 silicon–oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall formula SiO2.
There are many different varieties of quartz, several of which are semi-precious gemstones. Especially in Europe and the Middle East, varieties of quartz have been since antiquity the most commonly used minerals in the making of jewelry and hardstone carvings.
The word "quartz" is derived from the German word "quarz", which was imported from Middle High German, "twarc", which originated in Slavic (cf. Czech tvrdy ("hard"), Polish twardy ("hard"), Russian твёрдый ("hard")), from Old Church Slavonic тврьдъ ("firm"), from Proto-Slavic *tvьrdъ
Quartz belongs to the trigonal crystal system. The ideal crystal shape is a six-sided prism terminating with six-sided pyramids at each end. In nature quartz crystals are often twinned, distorted, or so intergrown with adjacent crystals of quartz or other minerals as to only show part of this shape, or to lack obvious crystal faces altogether and appear massive. Well-formed crystals typically form in a 'bed' that has unconstrained growth into a void, but because the crystals must be attached at the other end to a matrix, only one termination pyramid is present. A quartz geode is such a situation where the void is approximately spherical in shape, lined with a bed of crystals pointing inward.
At surface temperatures and pressures, quartz is the most stable form of silicon dioxide. Quartz will remain stable up to 573 °C at 1 kilobar of pressure. As the pressure increases the temperature at which quartz will lose stability also increases.
Above 1300 °C and at a pressure of approximately 35 kilobars, only β-quartz is stable. The latter is not the same as normal quartz (or α-quartz), low quartz or just quartz. β-quartz has higher symmetry, is less dense and has a slightly lower specific gravity. The conversion, from one solid substance to another solid substance, of quartz to β-quartz is quick, reversible and accompanied with a slight energy absorption. The conversion is so easily accomplished that when a crystal of quartz is heated to β-quartz, cooled back down, heated again to β-quartz, etc., the quartz will be the same as when it started.
The reason that the conversion is so easily accomplished is that the difference between quartz and β-quartz is relatively slight. The bonds between the oxygen and silicon atoms are "kinked" or bent in quartz and are not so "kinked" in β-quartz. At the higher temperatures the atoms move away from each other just enough to allow the bonds to unkink or straighten and produce the higher symmetry. As the temperature is lowered, the atoms close in on each other and the bonds must kink in order to be stable and this lowers the symmetry back down again.
Although all quartz at temperatures lower than 573 °C is low quartz, there are a few examples of crystals that obviously started out as β-quartz. Sometimes these are labeled as β-quartz but are actually examples of pseudomorphic or "falsely shaped" crystals more correctly labeled 'quartz after β-quartz'. These crystals are of higher symmetry than low quartz although low quartz can form similar crystals to them. They are composed of hexagonal dipyramids which are a pair of opposing six sided pyramids and the crystals lack prism faces. Quartz's typical termination is composed of two sets of three rhombic faces that can look like a six sided pyramid.
α-quartz crystallizes in the trigonal crystal system, space group P3121 and P3221 respectively. β-quartz belongs to the hexagonal system, space group P6221 and P6421, respectively.
These spacegroups are truly chiral (they each belong to the 11 enantiomorphous pairs). Both α-quartz and β-quartz are examples of chiral crystal structures composed of achiral building blocks (SiO4 tetrahedra in the present case). The transformation between α- and β-quartz only involves a comparatively minor rotation of the tetrahedra with respect to one another, without change in the way they are linked.
Pure quartz, traditionally called rock crystal (sometimes called clear quartz), is colorless and transparent (clear) or translucent, and has often been used for hardstone carvings, such as the Lothair Crystal. Common colored varieties include citrine, rose quartz, amethyst, smoky quartz, milky quartz, and others. Quartz goes by an array of different names. The most important distinction between types of quartz is that of macrocrystalline (individual crystals visible to the unaided eye) and the microcrystalline or cryptocrystalline varieties (aggregates of crystals visible only under high magnification). The cryptocrystalline varieties are either translucent or mostly opaque, while the transparent varieties tend to be macrocrystalline. Chalcedony is a cryptocrystalline form of silica consisting of fine intergrowths of both quartz, and its monoclinic polymorph moganite. Other opaque gemstone varieties of quartz, or mixed rocks including quartz, often including contrasting bands or patterns of color, are agate, sard, onyx, carnelian, heliotrope, and jasper.
Citrine is a variety of quartz whose color ranges from a pale yellow to brown. Natural citrines are rare; most commercial citrines are heat-treated amethysts or smoky quartzes. It is nearly impossible to tell cut citrine from yellow topaz visibly, but they differ in hardness. Citrine has ferric impurities, and is rarely found naturally. Brazil is the leading producer of citrine, with much of its production coming from the state of Rio Grande do Sul. The name is derived from Latin citrina which means "yellow" and is also the origin of the word "citron." Sometimes citrine and amethyst can be found together in the same crystal and is referred to as ametrine.
Citrine is one of three traditional birthstones for the month of November.
Rose quartz is a type of quartz which exhibits a pale pink to rose red hue. The color is usually considered as due to trace amounts of titanium, iron, or manganese, in the massive material. Some rose quartz contains microscopic rutile needles which produces an asterism in transmitted light. Recent X-ray diffraction studies suggest that the color is due to thin microscopic fibers of possibly dumortierite within the massive quartz.
In crystal form (rarely found) it is called pink quartz and its color is thought to be caused by trace amounts of phosphate or aluminium. The color in crystals is apparently photosensitive and subject to fading. The first crystals were found in a pegmatite found near Rumford, Maine, USA, but most crystals on the market come from Minas Gerais, Brazil.
Rose quartz is not popular as a gem – it is generally too clouded by impurities to be suitable for that purpose. Rose quartz is more often carved into figures such as people or hearts. Hearts are commonly found because rose quartz is pink and an affordable mineral.
Amethyst is a popular form of quartz that ranges from a bright to dark or dull purple color.
Smoky quartz is a gray, translucent version of quartz. It ranges in clarity from almost complete transparency to a brownish-gray crystal that is almost opaque. Some can also be black.
Milk quartz or milky quartz may be the most common variety of crystalline quartz and can be found almost anywhere. The white color may be caused by minute fluid inclusions of gas, liquid, or both, trapped during the crystal formation. The cloudiness caused by the inclusions effectively bars its use in most optical and quality gemstone applications.
Quartz is an essential constituent of granite and other felsic igneous rocks. It is very common in sedimentary rocks such as sandstone and shale and is also present in variable amounts as an accessory mineral in most carbonate rocks. It is also a common constituent of schist, gneiss, quartzite and other metamorphic rocks. Because of its resistance to weathering it is very common in stream sediments and in residual soils. Quartz, therefore, occupies the lowest potential to weather in the Goldich dissolution series.
Quartz occurs in hydrothermal veins as gangue along with ore minerals. Large crystals of quartz are found in pegmatites. Well-formed crystals may reach several meters in length and weigh hundreds of kilograms.
Naturally occurring quartz crystals of extremely high purity, necessary for the crucibles and other equipment used for growing silicon wafers in the semiconductor industry, are expensive and rare. A major mining location for high purity quartz is the Spruce Pine Gem Mine in Spruce Pine, North Carolina, United States.
ridymite and cristobalite are high-temperature polymorphs of SiO2 that occur in high-silica volcanic rocks. Coesite is a denser polymorph of quartz found in some meteorite impact sites and in metamorphic rocks formed at pressures greater than those typical of the Earth's crust. Stishovite is a yet denser and higher-pressure polymorph of quartz found in some meteorite impact sites. Lechatelierite is an amorphous silica glass SiO2 which is formed by lightning strikes in quartz sand.
The word "quartz" comes from the German About this sound Quarz which is of Slavic origin (Czech miners called it křemen). Other sources attribute the word's origin to the Saxon word Querkluftertz, meaning cross-vein ore.
Quartz is the most common material identified as the mystical substance maban in Australian Aboriginal mythology. It is found regularly in passage tomb cemeteries in Europe in a burial context, such as Newgrange or Carrowmore in the Republic of Ireland. The Irish word for quartz is grian cloch, which means 'stone of the sun'. Quartz was also used in Prehistoric Ireland, as well as many other countries, for stone tools; both vein quartz and rock crystal were knapped as part of the lithic technology of the prehistoric peoples.
While jade has been since earliest times the most prized semi-precious stone for carving in East Asia and Pre-Columbian America, in Europe and the Middle East the different varieties of quartz were the most commonly used for the various types of jewelry and hardstone carving, including engraved gems and cameo gems, rock crystal vases, and extravagant vessels. The tradition continued to produce objects that were very highly valued until the mid-19th century, when it largely fell from fashion except in jewelry. Cameo technique exploits the bands of color in onyx and other varieties.
Roman naturalist Pliny the Elder believed quartz to be water ice, permanently frozen after great lengths of time. (The word "crystal" comes from the Greek word κρύσταλλος, "ice".) He supported this idea by saying that quartz is found near glaciers in the Alps, but not on volcanic mountains, and that large quartz crystals were fashioned into spheres to cool the hands. He also knew of the ability of quartz to split light into a spectrum. This idea persisted until at least the 17th century.
In the 17th century, Nicolas Steno's study of quartz paved the way for modern crystallography. He discovered that no matter how distorted a quartz crystal, the long prism faces always made a perfect 60° angle.
Charles B. Sawyer invented the commercial quartz crystal manufacturing process in Cleveland, Ohio, United States. This initiated the transition from mined and cut quartz for electrical appliances to manufactured quartz.
Quartz's piezoelectric properties were discovered by Jacques and Pierre Curie in 1880. The quartz oscillator or resonator was first developed by Walter Guyton Cady in 1921. George Washington Pierce designed and patented quartz crystal oscillators in 1923. Warren Marrison created the first quartz oscillator clock based on the work of Cady and Pierce in 1927.
Quartz crystals have piezoelectric properties; they develop an electric potential upon the application of mechanical stress. An early use of this property of quartz crystals was in phonograph pickups. One of the most common piezoelectric uses of quartz today is as a crystal oscillator. The quartz clock is a familiar device using the mineral. The resonant frequency of a quartz crystal oscillator is changed by mechanically loading it, and this principle is used for very accurate measurements of very small mass changes in the quartz crystal microbalance and in thin-film thickness monitors.
Although many of the varietal names historically arose from the color of the mineral, current scientific naming schemes refer primarily to the microstructure of the mineral. Color is a secondary identifier for the cryptocrystalline minerals, although it is a primary identifier for the macrocrystalline varieties. This does not always hold true.
Chalcedony : Cryptocrystalline quartz and moganite mixture. The term is generally only used for white or lightly colored material. Otherwise more specific names are used.
Agate : Multi-colored, banded chalcedony, semi-translucent to translucent
Onyx : Agate where the bands are straight, parallel and consistent in size.
Jasper : Opaque cryptocrystalline quartz, typically red to brown
Aventurine : Translucent chalcedony with small inclusions (usually mica) that shimmer.
Tiger's Eye : Fibrous gold to red-brown colored quartz, exhibiting chatoyancy.
Rock crystal : Clear, colorless
Amethyst : Purple, transparent
Citrine : Yellow to reddish orange to brown, greenish yellow
Prasiolite : Mint green, transparent
Rose quartz : Pink, translucent, may display diasterism
Rutilated quartz : Contains acicular (needles) inclusions of rutile
Milk quartz : White, translucent to opaque, may display diasterism
Smoky quartz : Brown to gray, opaque
Carnelian : Reddish orange chalcedony, translucent
Dumortierite quartz : Contains large amounts of dumortierite crystals
From Wikipedia, the free encyclopedia
There are many different varieties of quartz, several of which are semi-precious gemstones. Especially in Europe and the Middle East, varieties of quartz have been since antiquity the most commonly used minerals in the making of jewelry and hardstone carvings.
The word "quartz" is derived from the German word "quarz", which was imported from Middle High German, "twarc", which originated in Slavic (cf. Czech tvrdy ("hard"), Polish twardy ("hard"), Russian твёрдый ("hard")), from Old Church Slavonic тврьдъ ("firm"), from Proto-Slavic *tvьrdъ
Crystal habit
At surface temperatures and pressures, quartz is the most stable form of silicon dioxide. Quartz will remain stable up to 573 °C at 1 kilobar of pressure. As the pressure increases the temperature at which quartz will lose stability also increases.
Although all quartz at temperatures lower than 573 °C is low quartz, there are a few examples of crystals that obviously started out as β-quartz. Sometimes these are labeled as β-quartz but are actually examples of pseudomorphic or "falsely shaped" crystals more correctly labeled 'quartz after β-quartz'. These crystals are of higher symmetry than low quartz although low quartz can form similar crystals to them. They are composed of hexagonal dipyramids which are a pair of opposing six sided pyramids and the crystals lack prism faces. Quartz's typical termination is composed of two sets of three rhombic faces that can look like a six sided pyramid.
α-quartz crystallizes in the trigonal crystal system, space group P3121 and P3221 respectively. β-quartz belongs to the hexagonal system, space group P6221 and P6421, respectively.
These spacegroups are truly chiral (they each belong to the 11 enantiomorphous pairs). Both α-quartz and β-quartz are examples of chiral crystal structures composed of achiral building blocks (SiO4 tetrahedra in the present case). The transformation between α- and β-quartz only involves a comparatively minor rotation of the tetrahedra with respect to one another, without change in the way they are linked.
Varieties (according to color)
Citrine
Citrine is one of three traditional birthstones for the month of November.
Rose quartz
Rose quartz is not popular as a gem – it is generally too clouded by impurities to be suitable for that purpose. Rose quartz is more often carved into figures such as people or hearts. Hearts are commonly found because rose quartz is pink and an affordable mineral.
Amethyst
Smoky quartz
Milky quartz
Occurrence
Naturally occurring quartz crystals of extremely high purity, necessary for the crucibles and other equipment used for growing silicon wafers in the semiconductor industry, are expensive and rare. A major mining location for high purity quartz is the Spruce Pine Gem Mine in Spruce Pine, North Carolina, United States.
Related silica minerals
History
While jade has been since earliest times the most prized semi-precious stone for carving in East Asia and Pre-Columbian America, in Europe and the Middle East the different varieties of quartz were the most commonly used for the various types of jewelry and hardstone carving, including engraved gems and cameo gems, rock crystal vases, and extravagant vessels. The tradition continued to produce objects that were very highly valued until the mid-19th century, when it largely fell from fashion except in jewelry. Cameo technique exploits the bands of color in onyx and other varieties.
Roman naturalist Pliny the Elder believed quartz to be water ice, permanently frozen after great lengths of time. (The word "crystal" comes from the Greek word κρύσταλλος, "ice".) He supported this idea by saying that quartz is found near glaciers in the Alps, but not on volcanic mountains, and that large quartz crystals were fashioned into spheres to cool the hands. He also knew of the ability of quartz to split light into a spectrum. This idea persisted until at least the 17th century.
Charles B. Sawyer invented the commercial quartz crystal manufacturing process in Cleveland, Ohio, United States. This initiated the transition from mined and cut quartz for electrical appliances to manufactured quartz.
Quartz's piezoelectric properties were discovered by Jacques and Pierre Curie in 1880. The quartz oscillator or resonator was first developed by Walter Guyton Cady in 1921. George Washington Pierce designed and patented quartz crystal oscillators in 1923. Warren Marrison created the first quartz oscillator clock based on the work of Cady and Pierce in 1927.
Piezoelectricity
Major varieties of quartz
Chalcedony : Cryptocrystalline quartz and moganite mixture. The term is generally only used for white or lightly colored material. Otherwise more specific names are used.
Agate : Multi-colored, banded chalcedony, semi-translucent to translucent
Onyx : Agate where the bands are straight, parallel and consistent in size.
Jasper : Opaque cryptocrystalline quartz, typically red to brown
Aventurine : Translucent chalcedony with small inclusions (usually mica) that shimmer.
Tiger's Eye : Fibrous gold to red-brown colored quartz, exhibiting chatoyancy.
Rock crystal : Clear, colorless
Amethyst : Purple, transparent
Citrine : Yellow to reddish orange to brown, greenish yellow
Prasiolite : Mint green, transparent
Rose quartz : Pink, translucent, may display diasterism
Rutilated quartz : Contains acicular (needles) inclusions of rutile
Milk quartz : White, translucent to opaque, may display diasterism
Smoky quartz : Brown to gray, opaque
Carnelian : Reddish orange chalcedony, translucent
Dumortierite quartz : Contains large amounts of dumortierite crystals
From Wikipedia, the free encyclopedia
