Most crystals grow underground. The growth process requires a large amount of groundwater containing saturated silicon dioxide, the temperature is between 550℃ and 600℃, and the pressure is two to three times higher than atmospheric pressure. After a long time, it becomes a hexagonal crystal.
It is a mineral when it is heated or squeezed, and it can easily become a liquid. It is also a very common rock-forming mineral, which can be found in all three types of rocks. Because it crystallizes at the latest in igneous rocks, it usually lacks a complete crystal plane, and most of it is filled between other rock-forming minerals that have crystallized before. The timely composition is the simplest silicon dioxide (sio2), which is glassy and has no cleavage plane, but conchoidal fracture. Microcrystal is called chalcedony, agate or jasper. Pure time is colorless, but it often contains impurities of transition elements, so it presents different colors. It is very stable in time and is not easy to weather or become other minerals. Silicon is located in the fourth group of the periodic table of elements, widely distributed in the earth's crust, and occupies the second place in the abundance distribution order of all elements, second only to oxygen. Silicon is also a typical oxygen-loving element, which mainly combines with oxygen to form silicon-oxygen tetrahedron SiO4- 4-4, and produces different silicate minerals through the combination of various forms of silicon-oxygen tetrahedron. Among gem minerals, silicate accounts for more than 80% and is distributed in the form of free silicon-silica. But also very stable, and it is the most common and important rock-forming mineral in nature. It is also a kind of gem with a large number of applications and a wide range in the jewelry industry. There are many kinds of gems with silica as the main component, each with its own characteristics. According to the crystallization degree of SiO2 _ 2, it can be divided into crystalline single crystal, polycrystalline quartzite jade, aphanitic chalcedony, agate, Australian jade, jasper, wood fossil, silicified wood, amorphous opal and natural glass. According to the national standards, they are described as follows: single crystal SiO22 _ 2 _ 2 single crystal (including twins) is transparent, and its crystal form is perfect. The mineral name is single crystal, that is, crystal in a broad sense, and crystal in a narrow sense refers to colorless and transparent varieties. The basic properties of (1) crystals belong to ternary system. The common crystal forms are columnar, and the main simplex forms are hexagonal columnar and rhombic. Stripes and polygon etching often appear on the cylindrical surface of columnar crystals. The crystal is uniaxial and has a unique bull's-eye interferogram, with a refractive index of 1.544- 1.553 and a birefringence of 0.009. Crystals are usually colorless and transparent, but many colors appear when they contain impurities. According to color, crystal can be divided into amethyst, topaz, smoky crystal and other varieties. (2) Variety and identification of crystals: colorless and transparent pure siliceous crystals may contain rich inclusions, such as negative crystals, fluid inclusions and solid inclusions. Negative crystal is an important basis for determining rock crystal. Rutile, tourmaline and actinolite, which are common in solid inclusions, are arranged in fine needle-like orientation in the crystal, just like hair, which is customarily called hair crystal. Other solid inclusions can form beautiful pictures in crystals and become attractive ornamental stones. Amethyst: A purple crystal, which is caused by trace iron in SiO2 _ 2. After irradiation, a pair of electrons in the electron shell of trivalent iron ions are excited, resulting in hole color center FeO 4+ stirring 4. The hole mainly absorbs visible light of 550nm, which makes the crystal purple. However, Fe3- is unstable when stirred, and it is easy to become trivalent iron when heated, so amethyst is easy to fade. It has weak to medium dichroism, and may have all inclusions in the crystal, as well as characteristic zebra stripes and spherical and droplet-like opaque dark inclusions.
Smoke crystal: a smoky to brown or even black crystal containing a small amount of aluminum. Al-3+ ion replaces Si-4+ ion, and after irradiation, AlO-4- 4 hole color center is produced, which makes the crystal smoke. Smoke crystals can become colorless crystals after heating.
Topaz: A yellow crystal containing trace amounts of iron. Topaz is generally transparent and has the same internal characteristics as amethyst. Most topaz in the market is made by heating amethyst.
Green crystal: a kind of green crystal, which is rarely produced naturally and is mainly obtained by heating amethyst; Or the crystal is colored by containing green minerals (such as chlorite).
Hibiscus stone: also known as Rose Seasons, it is light to medium pink crystal with light color, which is caused by trace manganese and titanium. Its single crystals are few, usually dense and massive, with cloudy and milky appearance, and sometimes it contains needle-like rutile inclusions arranged in a directional way, so it can show starlight when it is ground into a spherical gem.
Two-color crystal: a crystal with purple and yellow * * *. Purple and yellow occupy a part of the crystal block respectively, and there is a clear boundary between the two colors. The two colors are caused by the twin crystals in the crystal, and purple and yellow are developed on the R plane and Z plane of the twin monomer respectively. Quartz cat's eye: when the crystal contains a large number of parallel fibrous inclusions, the curved gem surface can show cat's eye effect. Generally, the curved surface of quartz cat's eye is high, and fibrous inclusions are clearly visible.
Starlight crystal: when there are more than two groups of needle-like and fibrous inclusions in the crystal, the surface of arc-shaped gemstone can show starlight effect, generally six stars or four stars. Polycrystalline silica jade is mainly composed of timely fine-grained jade, with a small amount of mica minerals, hematite and goethite. The aging time is a typical granular structure, the particle size is generally 0.0 1-0.6 mm, the aggregate is massive, slightly transparent to translucent, and the density is close to that of single crystal aging, ranging from 2.64-2.7 1g/cm ~ 3. The refractive index measured by the dot method is about 1.54, and the pure one is colorless, often colored because of containing fine colored mineral inclusions. Common varieties are: Dongling Stone: a kind of quartzite with sandstone effect. There is green Dongling Stone with chromite mica in the market, which is slightly transparent under the microscope and mainly produced in India. The timely particles are coarse, 0. 1-0.6 mm, and the flaky minerals contained in them are large and generally arranged in a directional way. It is reddish brown under the Charles filter.
Miyu: named after it is produced in Mixian County, Henan Province, it is a dense quartzite, containing 3 ~ 5% fine scaly sericite, mainly in green series, such as light green, emerald green and bean green. Compared with Dongling stone, the rice language is finer and denser, and its internal timely particle size is mainly 0.02~0.25mm, without obvious sandstone effect. At high magnification, fine green mica can be seen evenly distributed in the network. Guicui: Named after its production in Guizhou Province, it is a kind of fine-grained quartzite containing green kaolinite, which is uneven green with gray tone and is generally only used as a low-grade ornament.
Jing Baiyu: Named after its origin in the suburbs of Beijing, it is a white quartzite with fine texture, luster and oiliness. Sometimes it is used to pretend to be sheep fat white jade, which is distinguished by its low density and refractive index.
"Malaysian jade": It is a kind of green-dyed quartzite with fine structure, which is often used to pretend to be jadeite. Typical particle structure and relatively low refractive index can be easily distinguished from jadeite under amplification conditions. The national standard (GB/T 16553- 1996) has stipulated that quartzite should be used instead of this name. The aphanitic aggregate of aphanitic SiO2 _ 2 jade is fully bright and dense under orthogonal polarization, and it can also be spherical, radial or fine fibrous aggregate. The measured refractive index is 1.53, and the density is 6.5 ~ 7.0g/cm ~ 3. There are mainly four varieties of chalcedony, agate, jasper and Australian jade. Chalcedony: ultra-microscopic cryptocrystalline aggregate, the single crystal is fibrous, the intergranular pores are filled with water and bubbles, and the density is about 2.60g/cm 3. Because chalcedony is porous and easy to dye, the common bright chalcedony on the market is dyed. It is worth mentioning that the color of dyed chalcedony is relatively stable and it is also a low-grade jade. The national standard stipulates optimization and does not need to be explained.
Agate: chalcedony with banded structure. Sometimes the center of the banded structure is hollow, and sometimes it is filled with crystals. The most common natural colors of agate are white and gray, but also yellow-brown, reddish brown, blue and lavender. The basic properties of agate are the same as chalcedony. According to the characteristics of inclusions, the color distribution has the following special varieties.
Moss agate: It is a homogeneous translucent chalcedony, which contains dendritic green chlorite or black manganese oxide and red iron oxide. The impurities in the package are often moss-like, generally used as ornamental stones, also known as scenic agate, which is a valuable variety of agate.
Onyx: Also called banded agate, it is a kind of agate with single color and straight stripes. Usually used for stone carving and relief, common agate can have black and white stripes, or red and white stripes. When the onyx strip is as thin as silk, it is called silk-wrapped agate.
Water bile agate: It contains gas-liquid inclusions visible to the naked eye, and the agate gas-liquid inclusions will move when rotating.
Jasper: It's chalcedony with many impurities. The main impurity is iron oxide, so jasper is often red, but some are green, dark blue or black because there are other impurities. Jasper is opaque and dull. Stripes of different colors complement each other, just like a beautiful natural scenery, which is called landscape jasper. A kind of dark green jasper with red spots on it is called blood drop stone.
Australian jade: it is a kind of green chalcedony, which is green because of containing a small amount of nickel, with uniform color, transparent to translucent, mainly produced in Australia. Silica metasomatic jade is a kind of timely jade that is metasomatic by silica, but retains the original shape. The important varieties are wood stone and silicified wood. Wood-changed stone: It is the product that SiO2 _ 2 partially or completely explains glaucophane asbestos, but retains the crystalline form of fibrous asbestos, so it is named because its texture and color are similar to wood grain. Wood stone is opaque, with hardness of 6.5 ~ 7.0, density of 2.64 ~ 2.7 1g/cm 3 and refractive index of 1.54 ~ 1.55 (point). Colors are tan, brown, blue gray and blue-green. Blue is the color of residual asbestos in glaucophane, while yellow-brown and brown are caused by iron oxide-limonite. Wood stone can be divided into tiger eye stone, eagle eye stone and other varieties according to color.
Tiger eye stone is yellow and yellow-brown wood stone, and the surface of the finished product can have silky luster. When the fibers that make up the tiger's eye stone are fine and neatly arranged, the surface of the arc-shaped gem can produce cat's eye effect.
Hawkeye stone is mainly blue and gray-blue wood-changed stone, with insufficient replacement of SiO2 _ 2, and there are more asbestos residues in glaucophane.
Silicified wood: When SiO _ 2 accounts for the trunk buried millions of years ago and keeps the shape and fibrous structure of the trunk, the product is called silicified wood. The chemical composition is mainly SiO _ 2, which often contains impurities such as Fe and Ca, and its color is khaki, light yellow and yellowish brown. , is opaque. The hardness is 6.5 ~ 7.0, the density is 2.65 ~ 2.9 1g/cm, and the measured refractive index is 1.53. Bright colors, strong luster, clear wood structure and dense texture are preferred.