Corundum, derived from India, is a mineralogical name. There are three main variants of corundum Al2O3, which are α-Al2O3, β-Al2O3, and γ-Al2O3. Corundum is second only to diamond in hardness.
Corundum is mainly used for high-grade abrasive materials, bearing materials for watches and precision machinery. Ruby-based artificial crystal as a laser emitting material. Ruby and sapphire are both corundum minerals. In addition to the starlight effect, only translucent-transparent and colorful corundum can be used as gemstones. Red is called ruby, while corundum of other shades is collectively called sapphire commercially.
The name of corundum is derived from India, and it is the name of mineralogy. Gemologically, it is called ruby and sapphire. Myanmar, Sri Lanka, Thailand, Vietnam and Cambodia are the most important suppliers of high-quality rubies and sapphires in the world. Other producing countries include China, Australia, the United States and Tanzania.

White corundum
white corundum

Physicochemical properties

There are three main variants of corundum Al2O3, which are α-Al2O3, β-Al2O3, γ-Al2O3. According to X-ray diffraction analysis, there are also η-Al2O3 (equiaxed crystal system), ρ-Al2O3 (crystal System is uncertain), χ-Al2O3 (hexagonal system), κ-Al2O3 (hexagonal system), δ-Al2O3 (tetragonal system), θ-Al2O3 (monoclinic system). Corundum has various colors, including colorless, white, golden (pigment ion Ni, Cr), yellow (pigment ion Ni), red (pigment ion Cr), blue (pigment ion Ti, Fe), green (pigment ion Co, Ni , V), purple (Ti, Fe, Cr), brown, black (pigment ion Fe, Fe), blue-violet under incandescent lamp, red-violet effect under fluorescent lamp (pigment ion V).

Corundum is a kind of gemstone formed by crystallization of alumina (Al2O3). Corundum mixed with metallic chromium is bright red, generally called ruby; and blue or colorless corundum is generally classified as sapphire.

Corundum ranks 9th in the Mohs hardness scale. The specific gravity is 4.00 and it has a lattice structure of hexagonal cylinders. With the hardness of corundum and the relatively lower price than diamond, it has become a good material for sandpaper and abrasive tools.

Corundum has glass luster and hardness 9. The proportion is 3.95-4.10. It is formed under the conditions of high temperature aluminum-rich and silicon-depleted C, mainly related to magmatism, contact metamorphism and regional metamorphism.

Corundum is a man-made material made of bauxite as the main raw material and smelted in a mining furnace. It can be used as abrasives and refractory materials. The white with higher purity is called white corundum, and the one with small amount of impurities is called brown corundum.

95% Brown corundumbrown corundum

Chemical composition

It is Al2O3. Often contains trace impurities Cr, Ti, Fe, V, etc. The existence of these impurities is:

  1. Replace Al with quasi-homology.
  2. Oxides mixed in mechanically.

Crystal morphology

Corundum is formed under the conditions of high temperature aluminum rich and silicon poor. In endogenesis, it is produced in aluminum-rich, silicon-depleted magmatic rocks and pegmatites, which are symbiotic with feldspar and spinel. In metamorphism, those produced in gneiss are symbiotic with skarn, magnetite, muscovite, etc.; those produced in the contact zone between magmatic rock and limestone are symbiotic with calcite, magnetite, epidote, etc. The product of desiliconization of magmatic rock. It belongs to the trigonal crystal system, and the crystal form is usually in a perfect hexagonal column or barrel. Oblique stripes or horizontal stripes are often developed on the column surface, and sometimes triangular cracking patterns are seen on the bottom surface.


It is very rich, including almost all colors of red, orange, yellow, green, cyan, blue, and purple in the visible spectrum.
Pure corundum is colorless and presents different colors when it contains different trace elements.

Gloss and transparency

Transparent-opaque, the polished surface has bright glass luster or sub-adamant luster.

Black Corundumblack corundum

Refractive index

1.762-1.770 (+0.009, -0.005).

Double discount rate



They all have dichroism, and generally appear in different shades of color. Ruby and blue sapphire have strong dichroism, while other colors of sapphire are slightly weaker.


Under long and short wave ultraviolet light, ruby can find red fluorescence, and the intensity under long wave is higher than that under short wave, and sunlight can also excite its red fluorescence, but those with high Fe content have weaker fluorescence. Sapphires are generally non-fluorescent, but Cr-containing Sri Lanka and Montana sapphires sometimes show pink fluorescence. Some yellow sapphires from Sri Lanka may have apricot yellow or orange yellow fluorescence.


Corundum has a Mohs hardness of 9, which is the second highest hardness material found in nature so far, second only to diamond in hardness.


The specific gravity of most gem grade samples varies between 3.99-4.00. The content of impurity elements such as Cr and Fe affects the specific gravity value. The higher the content, the greater the specific gravity.

Melting point

Up to 2000℃-2030℃.

Application of corundum

General purpose

Because corundum has excellent high temperature properties and mechanical strength, it has been widely used in many industrial fields such as metallurgy, machinery, chemical industry, electronics, aviation and national defense. Its main uses are described below.

  1. Because of its high temperature resistance, corrosion resistance, high strength and other properties, it is used as a steel pouring nozzle, smelting rare precious metals, special alloys, high-purity metals, glass drawing, crucibles and utensils for making laser glass; various high-temperature furnaces, such as Linings (walls and tubes) of refractory materials, ceramics, iron-making blast furnaces; physical and chemical utensils, spark plugs, heat-resistant and oxidation-resistant coatings. Low-silicon sintered corundum brick with SiO2 less than 0.5% is a special lining for carbon black, boron chemical industry, fertilizer, synthetic ammonia reaction furnace and vaporization furnace.
  2. Due to the characteristics of high hardness, good wear resistance and high strength, it is used as a part of various reaction vessels and pipelines, chemical pumps in chemical systems; as mechanical parts, various molds, such as wire drawing dies, squeeze pencil cores Die mouth, etc.; used for cutting tools, abrasives, bulletproof materials, human joints, sealed grinding rings, etc.
  3. Due to the high temperature insulation, the sleeve tube and protection tube used as thermocouples are still used in atomic reactors with excellent insulation, coupled with low loss and low dielectric constant, and are widely used in the electronics industry Solid integrated electric furnace substrate tube base, shell, porcelain frame, slim window, missile radar antenna protective cover, etc.
  4. Al2O3 is an ionic crystal with a very stable structure. Used at high frequency, high pressure and high temperature, its insulation is still excellent, coupled with low loss and low dielectric constant, it is widely used in the electronics industry to squeeze solid circuit board bases, housings, porcelain Frame, micro-glass window, missile radar antenna protective cover, etc.
  5. Corundum products have good air tightness and are tightly airtight even at high temperatures, so they are widely used in electric vacuum, such as corundum for making various large electronic tube shells and double in-line package shells in solid microcircuits.
  6. Corundum thermal insulation materials, such as corundum light bricks, corundum hollow balls and fiber products, are widely used in furnace walls and roofs of various high-temperature furnaces, which are both high temperature resistant and thermally insulating.
  7. Transparent corundum products can be used to make lamps and microwave fairings. In addition, Na-b-Al2O3 products are electrolyte materials for the manufacture of sodium-sulfur batteries.

Characteristic use

As a kind of abrasive, corundum is widely used in abrasive abrasive tools. There are many types of corundum. The use of different types of corundum is also different. This column will introduce you to the characteristics and specific uses of corundum.

  1. Black Corundum
    It is a derivative of brown corundum and has a black appearance. The hardness is low but the toughness is good.
    It is mostly used for free grinding, such as grinding or rough grinding before electroplating of products, and also for making coated abrasives, resin cutting pieces, polishing blocks, etc.
  2. Sintered corundum
    It is made of fine materials of alumina or alumina powder, which is characterized by good toughness and can be made into abrasive particles of various special shapes and sizes.
    Mainly used for heavy-duty grinding steel ingot grinding wheels, suitable for wild grinding stainless steel ingots, etc.
  3. Zirconium corundum
    Its characteristic is that the hardness is slightly lower, but the toughness value is large, the strength is high, the crystal size is usually fine, and the wear resistance is good.
    Foreign zirconium corundum abrasives are mainly used for heavy-duty grinding, suitable for grinding heat-resistant alloy steel, titanium alloy and austenitic stainless steel.
  4. Chrome corundum
    It is a derivative of white corundum. Its appearance is rosy, its hardness is similar to that of white corundum, its toughness is slightly higher than that of white corundum, and it has high strength, good grinding performance and high grinding precision. Abrasives made with this abrasive have good shape retention.
    The scope of application is similar to that of white corundum, and is especially suitable for the fine grinding and forming grinding of various cutting tools, measuring tools and instrument parts. Generally, chrome corundum has better grinding performance than white corundum.
  5. Microcrystalline corundum
    It is a derivative of brown corundum. Its appearance, color and chemical composition are similar to brown corundum. It is characterized by small crystal size (50-280μm), good abrasive toughness, high strength and good self-sharpness.
    Suitable for grinding stainless steel, carbon steel, ball milled cast iron, etc. The grinding method is suitable for forming grinding, plunge grinding, precision grinding and heavy-duty grinding.
  6. Single crystal corundum
    The particles of single crystal corundum abrasive are composed of a single crystal, and have good polygonal cutting edges, high hardness and toughness, strong grinding ability, little grinding calorific value, the disadvantage is that the production cost is high, and there is waste gas in production , Wastewater is produced and the output is low.
    It can be used to grind hard and hard-to-grind metal materials such as stainless steel, high vanadium high-speed steel, heat-resistant alloy steel and workpieces that are easily deformed and burned. Considering the limitations of production conditions for single crystal corundum abrasives, generally only Recommended for grinding heat-resistant alloys and hard-to-grind metal materials.
  7. White corundum
    The hardness of white corundum is slightly higher than that of brown corundum, but the toughness is slightly poor. It is easy to cut into the workpiece when grinding. The self-sharpness is good, the calorific value is small, the grinding ability is strong, the efficiency is high, and the price is higher than that of brown corundum.
    Suitable for grinding steels with high hardness, such as high speed steel, high carbon steel, quenched steel, alloy steel, etc.
  8. Brown corundum
    The appearance of brown corundum is brown, with a content of 94.5%-97%. It has the characteristics of high hardness, high toughness, sharp particles, and relatively low price. It is suitable for grinding metals with high tensile strength. In the absence of other abrasives, it can generally be replaced by it.
    Widely used for rough grinding of ordinary steel materials, such as: carbon steel, general alloy steel, calcinable cast iron, hard bronze, etc. Brown corundum second grade abrasives are often used as milling wheels and resin cutting wheels. Sand tiles, emery cloth, sandpaper, etc.