Gemstone types related to weathering and mineralization are opal, turquoise, malachite, chlorophyll and (1) mechanical deposition: when the weathering products are washed and re-deposited by water, minerals with stable physical and chemical properties and high relative density are mechanically deposited and enriched, resulting in diamond placer, sapphire placer, ruby placer and crystal placer. Almost all kinds of gems are.
(2) Chemical deposition: direct crystallization from solution. In arid and hot climate conditions, in dry inland lakes, semi-closed lagoons and bays, various salt solutions crystallize due to supersaturation. Such as gypsum, anhydrite, halite, etc. Beautiful crystals can be used as ornamental stones.
(3) Biodeposition: the result of the action of biological organisms. It is often made up of biological bones and remains. Regional metamorphism is a large-scale metamorphism accompanied by regional tectonic movement. The main influencing factors of regional metamorphism are: temperature, pressure and fluid with H2O and CO2 as the main active components, which make the original rock minerals recrystallize, often accompanied by a certain degree of metasomatism, and form new minerals. Mineralization related to gems includes:
(1) Low-level regional metamorphism and mineralization: the formation of oh-containing silicate minerals such as serpentine and tremolite and serpentine jade deposits.
② Mid-term regional metamorphism and mineralization: gem deposits such as plagioclase, syenite, cordierite and diopside were formed.
(3) Advanced regional metamorphism and mineralization: garnet, cassiterite, corundum, spinel and other minerals without OH were formed, and gem deposits such as garnet, ruby and sapphire were formed. The temperature is 400-700℃ and the formation depth is about 3-8 km. Generally it can be divided into magmatic pegmatite mineralization and metamorphic pegmatite mineralization.
(1) Magmatic pegmatite mineralization is in the late stage of magmatism. Because the melt is rich in volatile components, it slowly crystallizes under the closed condition that the external pressure is greater than the internal pressure, forming coarse-grained minerals. Granite pegmatite has the most industrial value. The main minerals are feldspar, quartz, mica and rare and radioactive elements. The gems formed are beryl, tourmaline, topaz, crystal and so on.
(2) Metamorphic pegmatite is related to metamorphism and is the product of pegmatization in the late migmatization. But the industrial value is not great. Contact metasomatism mainly occurs in the contact zone between intermediate-acid magmatic rocks and carbonate rocks. Under the action of magmatic hydrothermal solution, magmatic rocks and carbonate rocks exchange chemical components, and various silicate minerals of magnesium, calcium and iron are formed in the contact zone, forming magnesium or calcium skarn. When the crystallization conditions are favorable, coarse-grained minerals can be formed and become gem raw materials.
(1) Mg skarn: Magma intrudes into dolomite or dolomite limestone, and the main gem minerals are forsterite, spinel, diopside, Mg-Al garnet, etc.
(2) Calcareous skarn: Magma is mainly formed by limestone surrounding rocks, and the main gem minerals are almandine, andradite, diopside, andalusite and Fushan stone. There are many sources of hydrothermal solution: post-magmatic hydrothermal solution, volcanic hydrothermal solution, metamorphic hydrothermal solution and groundwater hydrothermal solution. Post-magmatic hydrothermal solution is closely related to gem deposits. Post-magmatic hydrothermal solution refers to ore-bearing volatile substances, mainly water, gradually accumulated during magmatic crystallization, which are divided into:
(1) High-temperature ore-forming hydrothermal solution: 300-500°C, and the main gem types formed are Yingshi, topaz, tourmaline and beryl.
(2) Medium-temperature ore-forming hydrothermal solution: 200-300°C, and the main gem types formed are timely and agate.
(3) Low-temperature ore-forming hydrothermal solution: 50-200 C, and the main gem types formed are Yingshi, opal and emerald. Diamonds are precious to those who are colorless, transparent and brilliant; Emerald, ruby and sapphire, etc. , which enjoys a high reputation for its gorgeous colors. In the international gem circle, gems other than diamonds are collectively called colored gems. This is not only because of the difference in value and grade, but also because diamonds have stricter and more specific requirements than colored gems in terms of gem quality evaluation and technical indicators (such as color classification and lathe standardization).
Mineral materials that can be used as gems, especially high-grade and middle-grade gems, are not completely natural products. Many of them are synthetic minerals and imitations, which have the same chemical composition, crystal structure and physical and chemical properties as natural gem minerals. There are about 20 kinds of high and middle grade gem minerals in the international gem market, which are listed as follows according to Mohs hardness.
The gem of a diamond is called a diamond. Because of its high hardness (Mohs hardness 10), high refractive index (N=2.4 17) and strong dispersion (O.044), it is bright and hard. It is known as the "King of Gems" and is the most precious gem.
Diamonds are usually yellow, brown, blue, green and pink, but colorless ones are more popular. There are 10 extra-large gem-grade diamonds weighing more than 620 carats (124 grams) in the world, among which the largest Cullinan weighs 3 106 carats (62 1.35 grams) and has a size of 5×6.5×65438. China Changlin diamond, weighing 158.786 carats, was discovered in Linshu county, Shandong province on 1977 and is listed as a world-famous diamond. The main diamond producing areas in the world are Australia, Zaire, Botswana, the former Soviet Union, South Africa, Brazil, Namibia, Ghana, Central Africa, Sierra Leone and China. The Mohs hardness of corundum is 9, and the gems include ruby, sapphire, star ruby and Baoguang sapphire, all of which are precious gems.
Ruby is bright red because it contains chromium oxide, and pigeon blood red is precious. Sapphire is a general term for corundum gems of all colors except red (including colorless). The color of sapphire is related to the oxide content of iron and titanium. Brilliant blue and sky blue are the top grades, and cornflower blue is the best.
Ruby is mainly produced in Myanmar, Sri Lanka, Pakistan, Tanzania and Thailand. Sapphire is mainly produced in Australia, Myanmar, Cambodia, Kashmir, Sri Lanka, Thailand and the United States, and also in China. The largest ruby found at present weighs 3450 carats, and the largest star ruby weighs 138.7 carats, all of which are from Myanmar. The famous star sapphire "Star of India" weighs 536 carats and is produced in Sri Lanka. Emerald's Mohs hardness is 8.5, and gemstones include cat's eye and metamorphic stone: both of them are precious stones.
Cat's eye is an emerald variety, which is honey yellow to brownish yellow, slightly transparent to translucent, with fine needles and tubular contents (arranged parallel to the C axis), and is famous for its charming cat's eye optical effect on its curved polished surface. Other gem minerals with cat's eye phenomenon in nature include aquamarine cat's eye and tourmaline cat's eye. But the golden emerald cat's eye is the best and most precious, because it is a specialty of Sri Lanka (formerly known as Ceylon), so it is also called Ceylon cat's eye.
Metamorphic rock, also known as emerald, is an emerald containing a little chromium oxide, which is famous for its glassy green in the sun and red in incandescent lamp (or metamorphic rock effect). The main producing areas are Sri Lanka and the former Soviet Union, as well as Brazil, Myanmar and Zimbabwe. The name of beryl is emerald. Mohs hardness is 7.5 ~ 8, and beryl has different colors because it contains trace elements of different transition groups. Gem varieties are: emerald; Blue sky and blue-green aquamarine; Bright emeralds of red and rose; Golden turquoise: colorless turquoise.
Emerald is the most precious among emeralds, and is known as the "king of emeralds". The most famous producing area is Colombia, along with Brazil, the former Soviet Union, South Africa, Afghanistan, Zambia, Zimbabwe, India and Pakistan. The largest emerald crystal found weighs 24,000 carats and was found in South Africa in 1956.
Aquamarine is also a valuable variety, with sky blue as the top grade. The largest aquamarine crystal found in the world weighs 1 10.5 kg and is 48.5 cm long. 19 10 was discovered in Brazil. Brazil is also a major producer of high-quality aquamarine in the world. The United States, the former Soviet Union, China, Madagascar, Pakistan and India also have output. Zircon moss hardness is 7 ~ 7.5. The main gemstone varieties are red zircon, blue zircon and colorless zircon. Colorless and transparent zircon can be used as a substitute for diamond because of its high dispersion (0.039).
The largest blue-green zircon gem found in the world weighs 208 carats and comes from Sri Lanka. Gem-grade zircon is mainly produced in Indochina countries such as Thailand, Australia, Tanzania, Norway and China. Tourmaline is called tourmaline. Mohs hardness is 7 ~ 7.5. Gem varieties include pink tourmaline, blue tourmaline, green tourmaline, bicolor-multicolor tourmaline, tourmaline cat's eye and so on, among which red and blue varieties are more precious.
The known largest tourmaline crystal weighs 12kg, is long 130cm, and has a columnar diameter of 40cm. It was discovered in Brazil by 1978. The main producing areas are Brazil, Sri Lanka, USA, former Soviet Union, Myanmar, Tanzania and Madagascar. The Mohs hardness of garnet is 6.5 ~ 7.5. Pomegranate, a gem variety, presents various gorgeous colors due to different end-member components and transition element impurities, among which bright red and light green are the best.
The main gem minerals or varieties of garnet are: blood-red to fuchsia garnet, fuchsia to brownish-red garnet, rose to fuchsia garnet, green to emerald garnet, orange to brownish-red garnet, green or pink hydrogrossular, beeswax yellow to orange garnet, emerald garnet, yellow to yellowish green garnet, black garnet and emerald green. High-quality garnet enjoys the reputation of "Ural Emerald". The Czech Republic and Slovakia, the former Soviet Union, the United States, Kenya, Tanzania, Sri Lanka, Brazil, India and China all have production. Nephrite is a microscopic fiber aggregate of tremolite-actinolite series minerals. Mohs hardness 5 ~ 6, strong toughness.
Nephrite is a precious jade species second only to jadeite. It has won the reputation of auspiciousness, purity and nobility for its gentleness and simplicity. The development and utilization of nephrite in China has a history of nearly 3000 years, and the exquisite jade carving art is a treasure of the East. The main varieties are white jade, sapphire, jasper, topaz and jet, and suet white jade is a precious variety. China, Xinjiang and Taiwan Province Province are rich in nephrite, especially Hetian jade in Xinjiang. The former Soviet Union, Canada, New Zealand, Australia and the United States also produce xonotlite, which is a monoclinic silicate mineral containing potassium, calcium and sodium, mostly aphanitic blocks with Mohs hardness of 5 ~ 6. Gem-grade minerals are lavender and purplish red, with fine texture and clear patterns, which are high-quality jade; Pure transparent to translucent can be used as gems.
National Classification Standard of Gemstones in China
In July 2003, People's Republic of China (PRC) promulgated and implemented the national standard GB/T 16552-2003, and the laws and regulations regulating gemstones have come into effect. In the vast world of nature, among more than 3 100 substances, minerals and rocks that have been discovered and named by human beings, there are more than 30 kinds of 130 that can be made into ornaments. We call these beautiful stones gems, and now list their types and classify them:
Natural gems are produced by nature, water and air. They must have the following characteristics: certain aesthetics, certain acid and alkali resistance, corrosion resistance, rare and precious output, strong plasticity, and can be carved into decorative ornaments and works of art.
Natural jade is produced by nature, water and air, and they must also have the following characteristics: aggregate mineral objects with certain aesthetic appreciation, acid and alkali resistance, corrosion resistance, collection value and technological value, in which amorphous bodies account for a small part. For example: jade, white jade, Dongling jade, jasper, Hetian jade, obsidian, agate, opal and so on.
Naturally occurring organic gems are all formed by animals and plants on the earth. Like natural jade, this process sometimes takes hundreds of millions of years. A very small part of the organic matter formed has the characteristics of ordinary gems and can be made into various jewelry and decorations in life. This substance is called natural organic gem. Like dinosaur fossils, bones of marine life, pearls and shells, ivory carvings, seeds and stones of trees and so on.
Scientifically cultivated gemstones artificially cultivated by scientific and technological means are cultivated, maintained and grown entirely or partially by human factors of scientific and technological processes, and their chemical composition and physical properties are exactly the same as those of natural gemstones, such as cultured pearls and cultured corals.
Man-made products, or imitation gems, are made of other substances similar to jewelry and jade in appearance, color, optical effect, weight and density, and their chemical and physical components are completely different from natural gems. Such as artificial jade, artificial amber, artificial pearls and so on. This kind of material regarded as gem is basically glass, plastic, resin, clay and so on. , which mainly reflects the brand value of the process.
Reproduction of artificial products refers to the jewelry and jade articles made by human beings by using various technologies and processes, which are useless or inferior in the form of powder and slag, after melting, kneading and pressing. For example: amber, coral, turquoise, etc.