An important source of precious material is secondary sediments such as sand and gravel. They contain not only minerals deposited as a result of erosion of primary erupted rocks, but also minerals such as sapphire, ruby, spinel, brought from metamorphic rocks. The alluvium is used to mine a significant amount of diamonds, but in addition to this precious mineral, productive gravel probably accounts for about half of the total gemstone production. Sand and gravel are varieties of sedimentary rocks — rocks formed as a result of weathering of primary rocks, their transfer, and accumulation by water streams. In addition to the insoluble material, which is transported in suspension and deposited mechanically, the solution also contains soluble rock fracture products, which eventually crystallize and settle to form a chemical precipitates, such as gypsum and some types of limestone.
Some of the insoluble material can be accumulated as a result of living organisms: coral growth and the formation of pearls. Magmatic activity is associated with another important way of forming precious material. When a large mass of molten magma moves in the host sedimentary rocks of the Earth"s crust, the latter is heated up and undergo contact metamorphism, in which most or all of them experience recrystallization. Gemstones formed in the process of metamorphisms, such as ruby and spinel, may be fully composed of the material present in the primary rock; ruby is corundum formed from clay impurities in limestone; magnesium oxide, which is part of the spinel, is formed from primary dolomite, calcium-magnesium carbonate.
Frequently, volatile components that participate in the formation of new minerals, such as lapis-lazuli, penetrate into the host rocks from the intrusive body. The volatile components can also interact with the erupted rocks themselves; olivine is transformed into serpentine, and materials such as limonite, opal and agate can be deposited in cracks and voids. Contact metamorphism is usually local and confined to a small space along the area of interaction of the intrusive body with surrounding rocks, but under certain conditions, as a result of regional metamorphism, recrystallization of surrounding rocks occurs over large areas.
Thus, the formation of bedrock gemstones may be due to the following processes:
(a) Crystallization from deep basic magmas rich in magnesium and iron (diamond, pyrope, etc. ); b) crystallization in granite pegmatites of different genetic types (beryl, emerald, aquamarine, zircon, topaz, tourmaline, amazonite, etc. ); c) crystallization from hot and cold mineralized aqueous solutions of deep or surface origin (opal, rock crystal, amethyst, turquoise, agates, malachite, etc. ); d) formation of metamorphic and contact-metasomatic deposits (ruby, sapphire, spinel, lapis lazuli, garnets, jadeite, etc. ). Often, the gemstone mining sites are associated with placers that occur during the destruction of bedrock, in which these minerals were formed. Some of the precious ones are of organogenic origin (pearls, amber).
3 Deposits of gemstones Deposits of self-precious stones of various genetic types are widely spread all over the world. Russia has long been famous for its gemstone deposits. Wonderful deposits of colored jasper in the Urals and Altai give the best in the world in terms of the beauty of the stone and the size of the blocks of materials for large works of art. Green malachite and red eagle with a beautiful black pattern from the deposits of the Urals, jade from Eastern Sayan, dark blue lazuli from Baikal — these are the best in the world in beauty and technical merits of the stones. A huge number of marbles of various colors and patterns are discovered and mined in the deposits of Ukraine, the Urals, Karelia, the Caucasus and Transcaucasia, Central Asia, Eastern, and Western Siberia. Marble onyx of beautiful patterns and colors is available in deposits of Armenia and Uzbekistan, agate and chalcedony deposits in Transcaucasia and Eastern Siberia. No less known are the blue ones, the colors of seawater "Siberian aquamarines" from the deposits of the Urals and Eastern Siberia, the Urals emerald, different shades of topaz and amethyst of the Urals are considered to be the best in color and quality stones.
The world"s largest amber deposits are located in the Kaliningrad region. Turquoise deposits have been discovered in Central Asia, diamonds in the Urals and Yakut deposits, fire opal in Kazakhstan, and numerous deposits of rock crystal cut in the Urals, Kazakhstan, the Pamirs, and Eastern Siberia. Also, valued are Ural alexandrite — green in the afternoon and wine red in artificial light; bright pink "Sibirit" - tourmaline of the Ural and Siberian deposits; brilliant, like diamonds, phenacites; emerald-green garnet-demantoid. Deposits of gemstones are known in Africa, South America, Asia, Europe, and Australia. Their mining is very unevenly distributed.