Corundum, whose English name is Corundum, comes from Indian language kurand or kuruvinda, which is a mineral name, followed by impure corundum; Others think it comes from Tamil kurundum and Sanskrit kuruvinda, meaning ruby.
Ruby in English comes from Latin ruber, which means "red" and was translated as "thorn" in ancient China. In Sanskrit, ruby has many beautiful names, such as ratnaraj (king of gems) and ratnanayaka (crown of gems), which shows that Indians cherished it at that time. Sapphire is sapphire in English, which comes from the Latin Greek sapphire, meaning blue. In ancient China, sapphire was also called "Se Se" or "Sapphire", which was a transliteration of English.
The names of rubies and sapphires in Chinese come from their colors. In the Yuan Dynasty, Tao's Record of Dropping Farms contains the sayings of Hongya Valley, Qingya Valley, Huangya Valley and Baiya Valley. Some people think it may refer to red, blue, yellow and white corundum gems. Yagu is a transliteration of Arabian gems. In addition, in the ancient literature of China, there are words such as "light bead", "red" and "blue", which refer to red or blue gems, including corundum red and sapphire.
Ruby and sapphire are hard, second only to diamonds. Ruby is bright red and gorgeous, which is the crown of ruby and the first in colored gems. Sapphire is blue in color, or as fresh and vast as the sky after rain, or as broad and deep as the sea. It is also called the king of sapphire. Therefore, rubies and sapphires have been deeply loved by people and have a long history as ornaments. In the "top hat" of Qing officials in China, the prince used ruby for the first official and sapphire for the third official, which objectively reflected his status as a symbol of power and people's favor for it. Nowadays, all over the world regard ruby as the birthday stone in July and the 40th wedding anniversary stone (ruby wedding), which symbolizes love, enthusiasm and noble morality. People also call it "the stone of love". Sapphire is the birthday stone in September and the 45th anniversary stone (sapphire wedding), which symbolizes loyalty and loyalty.
First, the basic characteristics of corundum gem
Mineral name: corundum
Chemical composition: theoretical molecular formula Al2O3, which can contain many isomorphic impurities or mechanical inclusions. Isomorphic components, especially iron group elements replacing aluminum, such as chromium, iron, titanium, vanadium, cobalt, etc. , has an important influence on the color of gemstones, while dissolved substances-filaments and gas and liquid inclusions such as fingerprints and feathers decomposed from isomorphic components have a great influence on the purity and transparency of corundum gemstones.
Crystal system and crystallization habit: corundum belongs to the tripartite crystal system, usually in the form of column, barrel, waist drum, double cone and plate (figure 16- 1- 1), and multi-slice twins can be seen.
Optical properties: corundum can appear in many colors, including red and blue, yellow, purple, orange, green, brown, gray, black and colorless. The color of corundum is mainly caused by iron pigment ions such as chromium, iron, titanium, vanadium, cobalt and nickel replacing aluminum. Table 16- 1- 1 lists the color ions and contents of some colored corundum. Some sapphires also have a color-changing effect, such as the color-changing sapphires from Thailand, Myanmar and Sri Lanka, which are blue-green or blue-purple with gray tones in the sun and purple or purple in the incandescent lamp. Discoloration is caused by trace amounts of chromium, iron and titanium. However, the color-changing sapphire in Umba Valley of Tanzania (and the color-changing sapphire synthesized by Werner's method) contains V, which leads to the color-changing effect.
Figure16-1-/ideal morphology of corundum
Upper behavior single crystal; The lower row is double crystal: left double crystal plane ∨ () right double crystal plane ∨( 000 1).
Corundum is glass luster or diamond luster (when polished); Transparent to opaque; Negative optical properties of uniaxial crystals: Refractive index no:1.767 ~1.771,ne:1.759 ~1.763; Birefringence 0.008; The deviation is 0.0 18. Polychromatic colors are medium to strong, and the hue and intensity of polychromatic colors can be different for the same gemstone variety from different places of origin or different grades from the same place of origin. Generally, rubies are purplish red-orange red, sapphires are purplish blue-green or blue-green, and yellow sapphires are weak in polychromatic color.
The luminescence of corundum gemstones is very different, which is not only related to the type and content of isomorphic components in corundum, but also affected by the type and distribution of solid inclusions (including secondary alteration). Generally speaking, ruby shows weak, moderate to strong red or orange-red fluorescence under long-wave (366nm) and short-wave (254nm) ultraviolet light, but the fluorescence under short-wave is often weak. Sapphire generally doesn't shine. Sapphire and green sapphire from individual origin (Cambodia, Australia, Thailand, etc.). ) shows faint chalky blue to green fluorescence, while chrome-containing sapphires produced in Sri Lanka and Montana, USA also show red fluorescence with pink tones. X-rays also showed no or moderate strong red or orange red fluorescence.
Table 16- 1- 1 the relationship between impurities and color in corundum
The absorption spectra of different varieties of ruby and sapphire are obviously different because of the different contents of chromium, iron and vanadium. The characteristic absorption lines of ruby are double lines (fluorescent lines) of 692.8nm and 694.2nm, absorption lines of 668nm and 659.2nm, and the incidental absorption lines are double lines of 468.5nm (weak) and 475nm and 476.5nm. Some people, especially Thai people, also have 45 1.5nm and 466.5 nm. Sapphire has an iron line with a characteristic absorption of 45 1.5nm, but the heat-treated Sri Lankan sapphire does not have this iron line. Deep blue varieties from Australia, Thailand, Cambodia and Nigeria may also have weak lines of 460nm and 470nm, synthetic sapphires have no or only weak lines of 45 1.5nm, iron-rich yellow sapphires have characteristic iron wires of 45 1.5nm, and iron-poor ones (such as those made in Sri Lanka) have weak or no wires; The yellow sapphire after heat treatment has only a complete absorption band of 400 nm ~ 450 nm. The yellow sapphire synthesized by flame melting method is mainly colored by Ni, and only a weak absorption line can be seen around 455nm, but if it contains chromium, chromium lines can also be seen. Orange sapphire, such as Umba in Tanzania, can show the absorption lines of Fe and Cr; There is no iron absorption line in Sri Lanka. The orange sapphire synthesized by flame melting method and Chatham flux method is colored with Ni and Cr without iron wire. The green sapphire is colored by Fe or the combination of Fe and Ti, and the iron absorption line can be seen. Flame-fused products are colored by Co, V and Ni, and there are absorption lines at 500nm, 530nm, 635nm and 690nm, but no iron lines. Color-changing sapphire shows typical absorption line spectra of ruby Cr and Fe; The product synthesized by flame melting method has a vanadium absorption line of 473nm, a broad absorption band centered at 580nm and a fluorescence line of 690nm.
Corundum gem has special optical effects such as starlight, cat's eye and discoloration. Six-shot starlight is more common, and occasionally you can see twelve-shot starlight. Starlight is produced because there are dissolved filaments with proper size and quantity in the gem, usually rutile, hematite and ilmenite, which are arranged in parallel along the {} or {} of corundum and cut into an arc, so there will be six starlight on the top surface of the gem. If two filaments are arranged in parallel along {} and parallel along {}, twelve starlight rays can be displayed. If the filaments are arranged in parallel in only one direction, the cat's eye effect will appear in the direction perpendicular to the extension of the filaments. The color-changing effect of corundum gem is caused by containing trace amounts of chromium, iron, titanium or vanadium.
Mechanical properties: Mohs hardness of corundum is 9; Density. There is no cleavage, often cleavage in the bottom or rhombic direction, and the fracture surface is shell-shaped or uneven. Most cleavage is caused by the distribution of dissolved impurity minerals along a specific surface network. For example, corundum from different mines in the United States shows that the exfoliated diaspore is distributed along the rhombic surface network, which leads to the cracking of the rhombic surface, while the black star sapphire from Li Wu, Thailand is dissolved by hematite distributed on the bottom surface network, which leads to the cracking of the bottom surface. Of course, there are also cracks caused by the double crystal effect.
Corundum is resistant to high temperature and fire, with a melting point of 2000 ~ 2030℃ and a boiling point of 2707 60℃. The thermal conductivity is 0.0600 ~ 0.0834 cal/cm℃ s), which is higher than that of spinel, garnet and topaz and lower than that of diamond, and is about 1/35 ~ 1/70 of diamond. Therefore, thermal conductivity meter is also one of the effective tools to identify corundum gems.
Corundum gem has high chemical stability and is resistant to common acid and alkali corrosion, but it is slightly soluble in boiling nitric acid or phosphoric acid heated to 300℃. Soluble in borax at 800 ~ 1000℃ and in potassium bisulfite at 400 ~ 600℃.
Microscopic characteristics: mainly refers to complex inclusions, including mineral crystals, bands, gas-liquid inclusions, microcracks and so on. There are more than a dozen mineral inclusions in rubies and sapphires, such as rutile, zircon, corundum, spinel, garnet, mica, ilmenite, hematite, calcite and apatite. In addition, banded or irregular spots, fingerprints and feather-like gas-liquid inclusions are also common. Ruby microcracks are also very common, as the saying goes, "Ten Hong Jiu Cracks". Ruby and sapphire from different places often have their own unique inclusions or combinations of inclusions, which is of great significance for distinguishing ruby and sapphire and even distinguishing their places of origin, but we should also be cautious in application and pay attention to the complexity of natural conditions. Synthetic corundum gemstones are different from natural gemstones in inclusion characteristics, such as curved ribbon (flame melting method, Czochralski method), seed crystal, platinum or copper alloy sheet, residual flux, bubbles, etc., which can be used as a basis for distinguishing them from natural gemstones.
The most typical characteristic inclusions of Burmese rubies are rutile filaments, which appear together with other authigenic or round particles such as spinel, corundum, apatite and zircon, or form dense clouds with * * * to form unique patterns. Hexagonal bands and "syrup" spiral patterns are also common, with polycrystalline twins developed and straight louver twins, and fingerprints and feather inclusions are rare. Myanmar sapphire also contains dense cloud-like rutile filaments, which can present starlight effect. Sometimes it forms an angle of 30 with the distribution direction of hematite, ilmenite and other filaments, which can show twelve starlight, but other mineral inclusions are few, rich in secondary liquid inclusions, and the color band is undeveloped.
Among the rubies and sapphires in Sri Lanka, rutile is slender, rare and has no cloud-like characteristics. It will disappear completely after heat treatment. There are also liquid inclusions such as fingerprint-like, feather-like, cobweb-like, net-like, and negative crystals with biconical main crystals, which are composed of highly prominent two-phase inclusions. The guest crystals can be zircon, mica and apatite. The most important solid inclusions are zircon covered with tiny cracks and halos. Most Sri Lankan gems are uneven in color, and colorless areas can appear in a large area locally. The core of its blue gem is often colorless, and the blue color is concentrated near the crystal plane (sometimes just a thin layer).
Some rubies produced in Thailand and Cambodia come from syngenetic deposits on the border between the two countries, with flaky twins, flat ribbons and growth lines, no rutile filaments, few other mineral inclusions and common fingerprint inclusions. These fingerprint inclusions can be cut by white needle-like diaspore and divided into some flag-like patterns. Some liquid inclusions surround negative crystals or opaque inclusions, forming "Saturn rings" or "disks". After heat treatment, this Saturn fingerprint will melt into glass due to crystals, and some gases will freeze on the glass-filled surface, which is mistaken for primary molten inclusions. The guest crystal in Saturn can be almandine, apatite, pyrrhotite, plagioclase, olivine or pyroxene.
Uneven color bands can be seen in Thai sapphires, and mineral inclusions are concentrated in blue color bands, which can reflect milky white light. The characteristic guest crystals are zircon, feldspar and pyrrhotite. Relatively thick fingerprint-like and feather-like inclusions can be seen, forming curved folds.
The color of sapphire in Cambodia is pure and uniform, and the characteristic object crystals are pyrochlore and plagioclase. Small droplets and comma-like inclusions are arranged along the growth structure of the main crystal; Liquid with red or brown spots around fingerprints.
The sapphire velvet-like luxury of Kashmir is due to the reflection of foggy inclusions composed of internal slots or empty pipes. Its characteristic inclusions are tourmaline, fused feldspar, zircon, pitchblende, stripes and dust particles.
Australian sapphires are mostly dark blue-black blue, and their important characteristics are that they contain dust-like molten inclusions, fingerprint inclusions, niobite, pyroxene, zircon and other crystals with obvious color bands. Some packages look like the tails of comets.
Sapphire in Montana, USA is beautiful in color and has a small colorless band. The characteristic mineral inclusions are garnet and spinel.
China Shandong sapphire is similar to Australian sapphire, and its inclusions are characterized by various types of molten inclusions and minerals, such as zircon, feldspar, mica and columbite-tantalite.
Sapphire produced in Thailand, Australia and Shandong, China can also contain two types of filaments in Myanmar sapphire.
Sapphires of other colors also have similar inclusion characteristics as red or sapphire from the same origin.
Second, the types of corundum gems
Corundum gemstones can be divided into ruby and sapphire.
(1) Ruby: According to the regulations of the International Association of colored gems (ICA), corundum gems with red as the main tone are called rubies, including pigeon blood red, magenta, orange red, brown red, pink and red with other tone components. When you name it, it's ruby, and you don't need to add adjectives. How the depth and purity of timbre fall within the scope of specific description.
(2) Sapphire: All corundum gems except ruby belong to sapphire. Sapphire other than blue should be named with a color prefix, such as yellow sapphire, purple sapphire and orange sapphire.
Ruby and sapphire with special optical effects, such as starlight, cat's eye and color-changing effect, are also named with prefixes or suffixes, such as star ruby, starlight sapphire, color-changing sapphire and ruby cat's eye.
In the classification of corundum gemstones, the color is the first grade, and the special optical effect is the second grade. Generally can be divided into the second level. The name should be simple and the description should be specific. Other tonal changes of some rubies and sapphires in the main color can be described in detail in the description, such as yellow-green, blue-purple, etc.
Third, the evaluation of corundum gems
Corundum is a kind of colored gems, and its evaluation is considered from the following aspects. Someone summed it up as "4C" and added "1T (transparency)".
(1) color: pigeon red is the first ruby, followed by rose red and pink; Sapphire is cornflower blue, followed by deep blue and light blue, then gorgeous green and yellow; The hue should not be too dark or too bright. The purer the color, the more saturated the better.
(2) Transparency: including clarity. The less inclusions, the smaller the transparency, the purer the color, and the brighter or brighter the better. There are more inclusions in corundum gems than diamonds, and the clarity classification can only be done by comparing the size, quantity and distribution of inclusions with the naked eye.
(3) Cutting: The requirements for perfect symmetry, regularity and smoothness are the same as those for diamonds, except that the upper-lower ratio and table size are looser than those for diamond cutting. The degree of flash and symmetry is related to the quality and shape of the original stone. Ruby and sapphire are often cut into finished products. In order to keep the carat weight, try to flash better. In the evaluation, we should consider the unique splendor and corresponding rarity of gems from each origin, and should not put gems in the first place.
(4) Weight: Other things being equal, the greater the weight of rubies and sapphires, the higher their value. Generally speaking, rubies are rarer than sapphires and are in greater demand. The ring surface can be made separately when it is not less than 0.3ct, and it is rare when it is more than 5ct, while sapphire needs more than 0.5ct.
Ruby and sapphire with starlight effect mainly depend on whether the starlight is clear and perfect. The high-quality star line with obvious, medium, uniform and unbroken starlight should be considered, followed by color and transparency.
4. Synthesize corundum gemstones and optimize corundum gemstones and corundum gemstone imitations.
At present, there are many methods to synthesize ruby and sapphire, except flame melting, ChuHrasky method, melting zone method, hydrothermal method, flux method and so on. Ruby and sapphire synthesized are of excellent quality, especially by flux method and hydrothermal method, and the synthesis conditions are closest to natural growth conditions. Therefore, the external characteristics and inclusions of various gemstone crystals synthesized by these methods are similar to those of natural gemstone crystals, which can be almost confused. The distinction between synthetic products of these methods and natural gems is a difficult problem even in front of professional appraisers.
At present, the improvement methods of ruby and sapphire are: heating, diffusion, irradiation, dyeing, filling, film covering and so on.
Heat treatment or roasting treatment is an optimization method. Heating corundum at a temperature far below its melting point can change the colors of rubies and sapphires (removing stray colors and evil colors, improving and curing high-quality colors) and increase their cleanliness (removing defects and improving transparency).
More than 95% rubies and sapphires are heat treated. In heat treatment, if you simply heat, you often need to bake at a higher temperature to improve the color, and sometimes the color is gray. At present, it is usually heated in a certain oxidation or reduction atmosphere to achieve the desired effect. Ruby with pink, purple or blue-green hue produced in Myanmar and Vietnam can obtain pure bright red after baking and produce obvious Xing Cai. Sapphire from Changle, Shandong, China has a large crystal, but its color is dark, grayish green and its transparency is poor. The blue sapphire with better color can be obtained by heat treatment, but the process technology needs to be improved to achieve the pure treatment effect without gray tone at all. Padma sapphires (magenta and pink orange) can also be obtained by heat treatment. Sri Lanka's "Guida" corundum (translucent ivory corundum) can be heated to obtain blue sapphire.
Diffusion (infiltration or surface infiltration): a chemical treatment method. Colorless or light-colored sapphires are buried in a powdered colorant and heated in a crucible to improve the color of the surface layer of the gemstone. Its price is1/5 ~110 of natural products. Faceted products and natural products can be distinguished by observing the color difference between the edge (deep) and the surface (shallow) by soaking.
Irradiation: A treatment method that uses high-energy particles such as γ-rays, X-rays, high-energy electrons, neutrons, protons, etc. to irradiate a gem, thus changing its color. Sometimes radiation and heating are combined. There are two common problems in irradiated gemstones: first, the color is unstable and easy to fade; Second, after radioactive irradiation, there will be residual radioactivity harmful to human body, which should be left for a period of time (3 months to half a year) until the residual radioactive dose decays to a level that human body can bear before it can be put on the market.
Dyeing: a method of boiling dyes and gems in water to deepen or change the color of gems. This law has a long history. Ruby is often treated like this. Ruby dyes easily because it has many cracks. Sri lankans use a kind of bark and branches to cook a pale yellow sapphire in water, which can turn the sapphire into golden yellow and then add wax to form a protective layer. The characteristics of dyed gemstones are that the color is darker, thicker and brighter only in the cracks, and the original color of the gemstone will appear when it is far away from the cracks. An experienced appraiser can easily recognize this feature under a magnifying glass.
Filling: It is a treatment method to inject substances with similar refractive index to gems with obvious cracks, such as paraffin, oil, synthetic resin, etc., to improve the purity of gems and eliminate uneven light refraction caused by cracks. And dye is often injected at the same time to change the color. Mainly used for rubies with more cracks. Easy to identify.
Coating: a method of coating colored substances on the surface of a gem to change its surface color and surface properties. Ruby and sapphire are not used much. In a case in China, a colorless Xing Cai sapphire was painted with red plastic and passed off as a Xing Cai ruby.
Inducing or changing inclusions: the method of changing inclusions in gemstones by electric erosion and heat treatment to improve the color of gemstones can also be attributed to heat treatment. For example, the heat treatment of sapphire in Shandong, China, is to oxidize the original black inclusions to achieve the discoloration of sapphire. Ruby and sapphire synthesized by Werner method have cracks after heat treatment. When the crack reaches the surface of the gem, this method can be used to produce "fingerprint" inclusions in the crack to impersonate natural rubies and sapphires (secondary treatment of flame-fused rubies and sapphires).
Heating and diffusion are commonly used in the above optimization methods, but their other applications are limited. However, the products that induce or change the packaging method are easy to achieve the purpose of confusing the fake with the genuine (synthetic gemstones pass off as natural gemstones), and gradually become popular. Their products can be found in the jewelry markets in the world, Chinese mainland, Hong Kong, Taiwan and Chinese mainland. Consumers should be careful when buying. The imitations of ruby and sapphire are mainly glass and synthetic cubic zirconia, which are obviously different from ruby and sapphire in physical properties such as polarization, refractive index and density, and are easy to distinguish.
Verb (abbreviation of verb) Identification of corundum gems
1. Difference from other similar-looking gems
The original stone can be distinguished from other kinds of gems by its crystal morphology or habit, hardness, cleavage or fracture characteristics. For corundum sand and gravel without any crystal habit, it can also be identified by density and thermal conductivity. For the finished torus or cutting piece, it is mainly identified by optical constants, such as refractive index, polychromatic property, density and so on. Red spinel, garnet, zircon, tourmaline, beryl and glass are easily mixed with ruby, while blue spinel, tourmaline, tanzanite, cordierite, topaz, beryl and glass are similar to sapphire, and there are also synthetic yttrium aluminum garnet (YAG), cubic zirconia (CZ) and rare earth glass. For varieties whose individual physical parameters are similar or partially overlapped with corundum, such as some red garnets and corundum with similar refractive index and density, they can be distinguished by polarization, multicolor, fluorescence and thermal conductivity. In a word, we can always choose one or several reliable and simple observation and testing methods for the difference between corundum gems and other kinds of gems by comprehensively utilizing their physical properties.
2. The difference between natural and synthetic corundum gems
There are many methods to synthesize corundum gems. Because natural products and synthetic products are corundum with the same physical properties, the distinction between natural stones and synthetic products mainly depends on the characteristics of inclusions and trace elements. It is very important to observe the types and combinations of guest crystals in inclusions and the characteristics of growth bands or stripes. Table 16- 1-2 lists the differences between natural corundum gems and main synthetic products.
Table 16- 1-2 Differences between natural corundum gemstones and main synthetic products
3. The difference between natural corundum gems and processed products
The main methods to improve rubies and sapphires are heat treatment, diffusion treatment and dyeing. Pure heat-treated gemstones can be sold as natural products without special instructions. The marks of heat treatment can be distinguished by the change of inclusions, which can be used as a reference for purity evaluation. Diffusion processors should be declared at the time of sale. The identification of diffusion products mainly adopts immersion method, that is, the gem to be identified is immersed in water (or other liquid) and the color difference between the edge and facet is observed. If the edge is darker than the color of the facet, it means that it is likely to be diffuse. This soaking method can also be used to distinguish the growth lamellar characteristics of natural stones and synthetic products. Dyeing treatment is easy to identify, and we can observe whether the color is mainly distributed along the cracks by magnifying method.
6. Brief introduction to the types, occurrences and genesis of corundum gem deposits.
The genesis of ruby and sapphire deposits can be divided into magma type, pegmatite type, metamorphic type and placer type.
1. Magmatic type
Ruby and sapphire are mainly produced in basic volcanic rocks such as alkaline basalt. Corundum crystallizes in the deep crust and is then brought to the surface by basaltic magma eruption. Most sapphires in the world are of this origin, such as sapphires in China (including deposits or occurrences in Shandong, Hainan and Fujian) and sapphires in New South Wales, Australia (whose output accounts for more than 50% of the world's sapphire production); Some producing areas also produce rubies, such as Hainan, Thailand, Cambodia, Laos and Vietnam.
The Jorge Valley sapphire deposit in Montana, USA, is the only alkaline-basic lamprophyre deposit. There is often a layer of fine-grained spinel on the surface of sapphire crystal, which indicates that corundum was crystallized from magma in the early stage of magma crystallization. With the rapid upwelling of magma, Al2O3, magnesium and iron jointly formed spinel floating on corundum.
The Haze Mountain ruby deposit in Australia is the only magmatic-metamorphic transitional deposit in plagioclase complex. Ruby formed in the deep crust is carried to the shallow by plagioclase and dispersed in the rock mass in the form of phenocrysts or megacrysts. When the rock mass intrudes into place, it will be folded and deformed. When the minerals are recrystallized, the ruby will also be recrystallized and grow, forming a plate-like crystal with excellent color.
2. Pegmatite type
Weng Baatar ruby and sapphire deposit is a typical pegmatite deposit. Corundum is produced in ordinary pegmatite, famous for its orange red, sky blue green, sky blue gray, brown yellow and brown.
3. Metamorphic rock types
It can also be divided into regional metamorphism, contact metasomatism and hydrothermal alteration. This genetic type was once the most important corundum gem deposit with the largest output, but now the output has retreated to magmatic type.
(1) regional metamorphic type: The most famous Maguhong (Blue) gem deposit in Myanmar belongs to this genetic type. It is produced in marble near granite. In the past, it was considered as a skarn type deposit. Later research shows that ruby and sapphire were enriched and crystallized by Al2O3 in limestone when limestone was transformed into marble, and there was no genetic relationship with subsequent granitic magmatism. Gumu ruby has been famous for its pigeon blood red variety since ancient times, and it was also famous for starlight blue and ruby in the 1930s. Generally 1 ~ 10 mm short column, sometimes up to 5cm. Jagdalek ruby deposit in Afghanistan also belongs to this type; Similar to this type are ruby deposits in Pamir, Russia and Hansa, Pakistan.
There are also regional metamorphic ruby and sapphire deposits in gneiss and schist in Sri Lanka, the United States and Xinjiang. In Akto County, Xinjiang, China, rubies and sapphires are distributed in sillimanite plagioclase gneiss or granulite, and corundum is wrapped in sillimanite and other aluminum-rich and silicon-poor minerals. The colors are mainly purplish blue and gray, and a few are lavender and purplish red.
(2) Contact metasomatism type: sapphire deposits such as Kangdishan in Sri Lanka belong to this type. The ore body is located in the inner contact zone between syenite and marble, that is, syenite body. Crystals are biconical, barrel-shaped, columnar, blue, sky-blue green and so on. (No black and brown), and it is the main producer of high-quality sapphires and colored sapphires in the world (Padparadscha Corundum).
The famous Kashmir sapphire mine also belongs to this type. The deposit occurs in the inner contact zone between granite pegmatite and dolomitic limestone (sapphire is produced in the feldspar of pegmatite) or the contact zone between actinolite and tremolite or between pegmatite and mica gneiss. corundum is considered to be formed by metasomatism of feldspar when the gas-generated hydrothermal solution reacts with pegmatite. The crystal is several centimeters long and produces sapphires in sky blue, blue, purple, green, orange and yellow. Among them, the "cornflower" sapphire with purple in blue is the most famous, and it is the representative of Kashmir sapphire. Some crystal nuclei are colorless and only appear blue near the crystal surface.
(3) Hydrothermal alteration type: Ruby and sapphire deposits in Tanga, Tanzania and Urals, Russia belong to this type. The deposit occurs in altered ultrabasic rocks. Corundum is formed in the dike composed of mica and plagioclase, which is the product of hydrothermal alteration. Similarly, there are corundum dikes such as Gangyushan in North Carolina, USA, and ruby and sapphire deposits in Africa and South Africa, Indian and China, Qinghai and Anhui. Corundum occurs in altered ultrabasic rocks or its marginal contact zone. In the lentils of corundum mica plagioclase in Qinghai, China, corundum is mostly dark rose red, sometimes blue, or sapphire in the crystal and ruby outside, which is translucent; Anhui is produced in corundum biotite plagioclase veins or lentils, and its crystal is light purple-rose red.
4. Alluvial ore
Because corundum has considerable stability, it is often enriched in placer. Placer is the main source of high-quality rubies and sapphires, and its economic value is much more important than primary ore. All the above genetic types of primary ore have corresponding secondary placer. There are residual deposits, slope deposits and alluvial deposits.
So far, rubies and sapphires entering the international market come from the following countries: Myanmar, Thailand, Cambodia, Kashmir, Pakistan, Sri Lanka, Australia, the United States, Namibia (opaque rubies), Colombia (sapphires and purple sapphires), Japan (transparent crystals), Scotland (sapphires), Tanzania (rubies and decorative gems) and Zimbabwe (sapphires of various colors)