The most 60% composition of crystal is "silicon dioxide", and the color of crystal is caused by various trace metals except silicon dioxide. In the natural environment, most crystals are "born" with calcite, pyrite, pyroxene, mica flakes of various colors, Biqian, granite, rutile and other minerals. , thus forming some scenes that are suspected to be true, that is, the so-called "different crystals", which increases the interest and value of collecting crystals.

Most of the crystals grow underground, and the growth process needs a lot 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 and widely distributed in the earth's crust, ranking second 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 a silicon-oxygen tetrahedron SiO4 _ 4- 4-4-4, and the silicon-oxygen tetrahedron combines in various forms to produce different silicate minerals. Among gem minerals, silicate accounts for more than 80%, with free silica; Silicon distributed in the form of SiO _ 2 also occupies an important position, and its stability is good. It is the most common and main rock-forming mineral in nature, and it is also a kind of gem with a large number and wide application range in jewelry industry. There are many kinds of gems with SiO _ 2 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, describe the following contents respectively:

1. Single crystal SiO22 _ 2 single crystal (including twins), the gem is transparent and the crystal form is perfect. The name of the mineral 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.

2. 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 similar to that of single crystal aging time, ranging from 2.64-2.7 1g/cm ~ 3. The measured refractive index is about 1.54, which is pure and colorless, often because it contains tiny particles. Common varieties are:

Dongling Stone: It is a kind of quartzite with the effect of sandstone. The green Dongling stone containing chromite mica is common in the market and slightly transparent under the microscope, mainly produced in India. The timely particles are coarse, 0. 1-0.6mm, and the flaky minerals contained in them are large, which are generally oriented. 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. It is uneven and gray, 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.

3. The aphanitic aggregate of aphanitic silica jade is fully bright and dense under orthogonal polarization, and it can also be spherical, radial or micro-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. The center of the ribbon structure is sometimes hollow and sometimes 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 kind of uniform 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.

4. Jade with silica replacement This is a timely jade that is replaced 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 brown and brown are iron oxides. It is caused by 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 ~ 3, and the measured refractive index is 1.53. Bright colors, strong luster, clear wood structure and dense texture are preferred.

5. Amorphous silica gems Amorphous silica gems include opal and natural glass.

(1) Opal originated from the Latin Opalus, which means the midpoint of "integrating the beauty of gems". Opal is now listed as the birthstone of October by the gem industry. Opal is a precious opal with color-changing effect, and its chemical composition is SiO _ 2·NH2O. Although it has no periodic structure peculiar to crystals, its internal structure is still orderly. Opal discoloration is a diffraction grating formed by SiO _ 2 spheres with the same diameter arranged regularly in three-dimensional space. Moreover, due to the different diameters of the balls, each cell will produce different spots. When the gem rotates, the incident angle of light will change, and the color of each spot will also change, that is, change color. Due to the different transparency, body color and color change form, it can be divided into three categories:

Black opal: It is a kind of opal, and its body color is black, including gray-black, dark blue and brown, and black is the most ideal. Due to the deep body color, the reflections of various colors are particularly magnificent, and the output of black opal is scarce, so its price is the highest among opal gems and it is one of the precious gems.

White opal: Opal that changes color on a white or light gray substrate, which is generally translucent and slightly discolored. It is the most common opal. Fire opal: transparent to translucent, sometimes discolored, sometimes not discolored, yellow to orange in color, which is loved by most Americans because of its warm and dynamic color.

Because opal contains moisture and its hardness is 5 ~ 6.5, it is generally used for necklace hanging, earrings and brooches, and is not suitable for ring face. When wearing, be careful not to expose to the sun or bake with fire, otherwise it will crack and fade easily.

(2) Natural glass: Natural glass refers to the glass formed under natural conditions, which is mainly composed of amorphous SiO2 and contains a small amount of Al2O3, FeO, Fe2O3, Na2O, K2O, etc. It has glass luster, opaque to translucent, and the cross polarizer is all black and isotropic, but it is usually wavy and abnormally extinguished. Zoom in and observe the common circular bubbles and flow structures inside. The refractive index measured by point method is 1.49 and the density is 2.33 ~ 2.46 g/cm 3, which is relatively stable. Natural glasses that can be used as gems are volcanic glass and meteorite glass.

Volcanic glass: It is the product of rapid condensation of acidic volcanic lava. The mineral name is obsidian, and the content of silica is 60 ~ 75%. It can be black, brown, gray, blue, yellow, red and so on. Sometimes the color is uneven with white or other mottled patches. It is called "Snowflake Obsidian" in the shape of snow.

Meteorite glass: it is a kind of natural glass caused by meteorites. It is formed by the rapid condensation of timely meteorites after falling into the atmosphere and burning. It is usually transparent green, greenish brown or brown.

By the way, introduce the identification of other gems:

Optimization and Identification of Diamonds

The optimization of diamonds mainly refers to improving those disliked colors (such as light yellow, light brown and brown) by various physical methods (radioactive irradiation and high temperature treatment) to get popular white or other colors (yellow, green, blue and red). Secondly, the inclusions in diamonds are treated by laser technology.

1. The Past and Present of Diamond Color Optimization

In fact, people have been optimizing the color of diamonds for a long time. In the past, the method used to improve the color of diamonds was very simple. For example, 1652, people know that when setting diamonds, thin foil is placed at the bottom to improve the color, or plant dyes and ink are applied to the surface or waist of diamonds to improve the color or improve the color level. 1905, British chemist William crookes discovered that diamonds buried in radium bromide can turn green. This is the beginning of discoloration caused by radioactive irradiation. By 1932, people finally found a safe and effective way to change the color of diamonds, while avoiding the harm of radioactivity to human body.

At present, the main ways to change color by irradiation are:

(1) Radium irradiation treatment (α particles) (faster coloring in radon gas);

(2) radiation treatment of artificially generated element americium (α particle); Irradiated diamonds can be strongly cleaned without leaving any radioactive traces;

(3) cyclotron therapy (proton, deuterium, alpha particle); Bombarding diamond with the particles which generate high-speed motion through cyclotron acceleration and coloring it;

(4) linear accelerator (high energy electron);

(5) nuclear reactor treatment (high energy neutrons);

Among them, the latter two kinds are commonly used, especially the diamond treated by nuclear reactor has a uniform color distribution. It is worth noting that when using the accelerator, the sample must be cooled in advance to prevent the diamond from suddenly heating up due to the heat generated by radiation, causing thermal oscillation and breaking the sample. Most of the treatment objects are type Ia, and the irradiation results are generally green and blue-green, and yellow-green, strong yellow, orange or orange-brown are obtained after reheating. The final result of processing a very small amount of type I diamonds may be pink or purple; The final treatment result of type II diamonds is brown.

Heat treatment is usually accompanied by irradiation treatment, and there are few cases of separate heat treatment. Previous studies have recorded that single heat treatment turns Ia-type diamonds into bright yellow, and Ia-type diamonds and Ib-type diamonds are transformed into each other under different conditions. The key of individual heat treatment is temperature control and atmosphere matching.

We studied Hunan placer diamond by absorption spectrum, electron paramagnetic vibration spectrum and infrared spectrum. The results show that the color centers of yellow, green and brown diamonds are lattice vacancies caused by impurity ions and radioactive irradiation. Solitary nitrogen center (≥ 2.22 ev) was found in the experiment. N3-N2 center (2.985 eV, 2.596 eV); GR 1 center (1.673 ev); ); 595 center (2.086 ev); H3 and H4 centers (2.463ev and 2.499 EV); 3H center (2.462ev). The color essence of diamonds in this area is due to the existence of joint color centers. The color of yellow or brown diamonds is due to the superposition of various color centers, which is basically consistent with the research results of Chen Feng and Guo Jiugao of Institute of Geochemistry, Chinese Academy of Sciences. Although the color of placer diamonds in this area is changeable, as long as the color mechanism is clear and the conditions such as temperature, pressure and atmosphere are well controlled, the grade of diamonds can be completely improved. At the same time, in the experiment, we found that there are hydrogen bonds in diamond, and the specific mode of existence (C-H, H2O, or OH) is still unclear. It is considered to be the third chromogenic impurity element in diamond except N and B, and related research work is under way.

CVD coating is a new technology to optimize diamond color. Generally, the crown of Ia faceted diamond is coated with a layer of blue artificial diamond film with a thickness of several to several tens of microns by chemical vapor deposition to imitate natural blue diamond.

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