Corun­dum is a min­er­al of the oxide class com­posed of alu­minum oxide (Al₂O₃). It is char­ac­ter­ized by extreme­ly high hard­ness (9 on the Mohs scale), sec­ond only to dia­mond. It may be col­or­less or exhib­it vivid col­ors due to impurities—such vari­eties as ruby (red) and sap­phire (blue and oth­er shades) are con­sid­ered gem­stones.

The name corun­dum orig­i­nates from the San­skrit word kuru­vin­da, mean­ing “ruby.” The term entered Euro­pean sci­en­tif­ic usage through Ara­bic and Per­sian medi­a­tion. As a min­er­al, corun­dum has been known since ancient times due to its trans­par­ent varieties—ruby and sapphire—which were high­ly val­ued in India, Bur­ma (Myan­mar), and Sri Lan­ka as pre­cious stones. In Europe, corun­dum as alu­minum oxide was described in the 18th cen­tu­ry, and from the 19th cen­tu­ry it began to be active­ly stud­ied as a tech­ni­cal material—initially nat­ur­al, and lat­er syn­thet­ic.

Corun­dum is includ­ed in the list of min­er­als of nation­al impor­tance, approved by Res­o­lu­tion of the Cab­i­net of Min­is­ters of Ukraine No. 827 of Decem­ber 12, 1994, as an abra­sive raw mate­r­i­al.

Corun­dum is a wide­spread min­er­al of the oxide and hydrox­ide class, con­sist­ing of alu­minum oxide (Al₂O₃) with a lay­ered struc­ture, and it has the high­est hard­ness after dia­mond (9 on the Mohs scale). Trans­par­ent vari­eties are gem­stones, while opaque ones are used as abra­sive mate­ri­als.

Corun­dum is char­ac­ter­ized by high chem­i­cal resis­tance and a high melt­ing point (2050 °C). Its col­or is typ­i­cal­ly bluish-gray or yel­low, although red vari­eties (ruby) and blue ones (sap­phire) also occur. Corun­dum crys­tal­lizes in the trig­o­nal sys­tem, form­ing bar­rel-shaped, pris­mat­ic, and tab­u­lar crys­tals.

Corun­dum ores are clas­si­fied into rich and poor types. There are no tech­ni­cal require­ments for low-grade ores. High-grade ores are divid­ed into three class­es, which must meet the fol­low­ing cri­te­ria: 1) by corun­dum con­tent: Class I — over 75%, Class II — 60–75%, Class III — over 55%; 2) by Al₂O₃ con­tent: Class I — over 80%, Class II — 65–80%, Class III — over 55%; 3) by Fe₂O₃ con­tent: Class I — up to 2%, Class II — 2–3%, Class III — up to 15%.

Corun­dum has sev­er­al vari­eties:

• Ruby — red corun­dum col­ored by chromi­um (Cr³⁺) impu­ri­ties. It is a first-cat­e­go­ry gem­stone, and the price of trans­par­ent, well-col­ored spec­i­mens can exceed that of dia­monds. “Pigeon blood” rubies from Myan­mar (Mogok Val­ley and Mong Hsu province) are the most valu­able.
• Sap­phire (“blue yakhont”) — blue in var­i­ous shades due to iron and tita­ni­um impu­ri­ties. Mod­er­ate­ly intense corn­flower-blue sap­phires are clas­si­fied as first-cat­e­go­ry gem­stones, though gen­er­al­ly less valu­able than rubies. A rare vari­ety is pad­parad­scha, a pink-orange sap­phire.
• Star ruby and star sap­phire — exhib­it aster­ism (a 3- or 6‑rayed star effect) caused by rutile inclu­sions.
• Emery — a dense, dark aggre­gate of corun­dum mixed with mag­netite, spinel, and oth­er min­er­als.
• Nazh­daq (emery rock) — a com­pact gran­u­lar corun­dum rock of gray-black col­or with impu­ri­ties such as hematite and mag­netite.
• Tech­ni­cal corun­dum — nat­ur­al or syn­thet­ic, used in grind­ing wheels, refrac­to­ry mate­ri­als, and ceram­ics.
• Syn­thet­ic corun­dum — pro­duced by ther­mal pro­cess­ing of var­i­ous high-alu­mi­na raw mate­ri­als.

Six main genet­ic types of corun­dum deposits are dis­tin­guished:

• Peg­matitic type, asso­ci­at­ed with veins and lens­es of corun­dum-bear­ing syen­ite peg­matites with­in mas­sifs of alka­line and nepheline syen­ites (Ilmen Moun­tains in the Urals, Khibiny, Myan­mar, Cana­da).
• Hydrother­mal type, rep­re­sent­ed by thick corun­dum and muscovite–corundum lens­es and nest-like bod­ies with­in sec­ondary quartzites (Sem­iz-Bugu in Kaza­khstan, Aktash in Cen­tral Asia).
• Pneu­ma­tolyt­ic-hydrother­mal type, asso­ci­at­ed with skarns devel­oped after mar­bles, as well as corun­dum-bear­ing pla­gio­cl­a­sites and mica­ceous rocks occur­ring in ultra­ba­sic rocks and calc-sil­i­cate for­ma­tions (Mogok in Myan­mar, Rai-Iz and Bor­zovske in the Urals, Kash­mir in India, etc.).
• Meta­mor­phogenic type, con­fined to high-alu­mi­na Pre­cam­bri­an schists, gneiss­es, and amphi­bo­lites, occur­ring as dis­tinct hori­zons enriched in corun­dum, kyan­ite, or sil­li­man­ite (Keivy deposit on the Kola Penin­su­la, Dra­gunske in Ukraine). Emery may also form dur­ing the meta­mor­phism of baux­ites (Samos and Nax­os islands in Greece, Namaqua­land in South Africa).
• Con­tact-meta­mor­phic type, rep­re­sent­ed by irreg­u­lar lens­es and nests of emery with­in mas­sifs of gab­bro, norites, and gran­ites, as well as at their con­tacts with host rocks and in recrys­tal­lized xeno­liths of high-alu­mi­na rocks (Sinan­goy deposit in Khakas­sia).
• Exoge­nous type, rep­re­sent­ed by elu­vial, delu­vial, and allu­vial plac­er deposits of corun­dum (Sem­iz-Bugu in Kaza­khstan); gem­stone-qual­i­ty mate­r­i­al is also extract­ed from plac­ers.

Lead­ing coun­tries in the pro­duc­tion of nat­ur­al corun­dum con­cen­trate include Turkey, Greece, the USA, India, Zim­bab­we, South Africa, Uruguay, and oth­ers. Gem-qual­i­ty corun­dum is main­ly extract­ed from plac­er deposits and less com­mon­ly from pri­ma­ry sources: rubies are mined in Myan­mar, Thai­land, Tan­za­nia, and Sri Lan­ka; sap­phires in Aus­tralia, Myan­mar, India, Cam­bo­dia, the USA, Thai­land, and Sri Lan­ka. In Kaza­khstan, the andalusite–corundum deposit Sem­iz-Bugu is being devel­oped, while in Uzbek­istan the Aktash and Cher­ak­say deposits are exploit­ed. In 1976, glob­al pro­duc­tion of tech­ni­cal corun­dum amount­ed to 8,551 tonnes, includ­ing 528 tonnes in India, 381 tonnes in Uruguay, and 252 tonnes in South Africa; pro­duc­tion has been increas­ing annu­al­ly by 1–2%.

Nat­ur­al corun­dum is not cur­rent­ly mined in Ukraine, although its occur­rences are known in high­ly meta­mor­phosed alu­mi­na-rich rocks of the Azov and Ingul (Kirovohrad) megablocks of the Ukrain­ian Shield.

In the Azov megablock, corun­dum occurs in corundum–sillimanite-bearing rocks. Its fore­cast resources are esti­mat­ed at 2.4 mil­lion tonnes. In East­ern Azov, corun­dum occur­rences have been iden­ti­fied near the vil­lages of Petriv­ka, Man­gush, and Demi­an­iv­ka, with con­tents of up to 1–5%. In Cen­tral Azov, the Tem­riuk area (R3 resources with 1–5% corun­dum total­ing 1,462 thou­sand tonnes), the Sado­vo-Bohdaniv­ka area (R3 resources with 8% corun­dum total­ing 213 thou­sand tonnes), and the Berdian­sk area are known.

In West­ern Azov, the Dra­gunske sillimanite–corundum deposit is known, com­pris­ing lay­ers of sil­li­man­ite- and corun­dum-bear­ing rocks (up to 20% corun­dum and up to 45% sil­li­man­ite) with thick­ness­es of 0.5–1.5 m. Corun­dum reserves to a depth of 100 m are esti­mat­ed in cat­e­gories A+B+C1 at 10 thou­sand tonnes, though they may sig­nif­i­cant­ly increase with fur­ther explo­ration of the deposit and its flanks. Resources of cat­e­go­ry P2 amount to 728 thou­sand tonnes to a depth of 200 m with an aver­age con­tent of 1%. Over­all fore­cast resources are esti­mat­ed at 9.5 mil­lion tonnes. Dur­ing devel­op­ment, corun­dum, sil­li­man­ite, gar­net, mica, and quartz–feldspar con­cen­trates would be recov­ered.

In the Ingul (Kirovohrad) megablock, corun­dum-bear­ing rocks are asso­ci­at­ed with the Kosharo-Olek­san­driv­ka suite (Koko­livske occur­rence, Kap­i­tan­s­ka, Solomi­ivs­ka, and Kosharo-Olek­san­driv­ka areas). Occur­rences are also known in the Chudniv–Berdychiv dis­trict (near the vil­lages of Musi­iv­ka and Plysne), asso­ci­at­ed with Berdy­chiv gran­ites, but the con­tent of corun­dum in them is very small and the prospects of these man­i­fes­ta­tions are unclear.

Corun­dum is used in the abra­sive indus­try, some­times as a refrac­to­ry mate­r­i­al, while noble vari­eties (ruby and sap­phire) are first-class gem­stones.

A dis­tinc­tion is made between abra­sive materials—crushed min­er­al sub­stances (grains, pow­ders) capa­ble of cut­ting, grind­ing, pol­ish­ing, and wearing—and abra­sive prod­ucts such as tools and instru­ments. The use of corun­dum as a raw mate­r­i­al for abra­sives is due to its high hard­ness and mechan­i­cal strength. Grain sizes range from sev­er­al mil­lime­ters to microm­e­ters.

Syn­thet­ic trans­par­ent corun­dum (sap­phire glass) is used in opti­cal win­dows for lasers and CCD sen­sors, in van­dal-resis­tant screens, and as pro­tec­tive glass for high-end watch­es and smart­phones.

Corun­dum is an extreme­ly hard and chem­i­cal­ly resis­tant min­er­al with wide indus­tri­al and gemo­log­i­cal sig­nif­i­cance. Its nat­ur­al vari­eties, such as ruby and sap­phire, are val­ued as first-class gem­stones, while tech­ni­cal and syn­thet­ic corun­dum are used in abra­sive, refrac­to­ry, and opti­cal indus­tries. In Ukraine, promis­ing occur­rences are known with­in the Ukrain­ian Shield (in the Azov region and the Mid­dle Dnieper area), with esti­mat­ed resources of up to 9.5 mil­lion tons, indi­cat­ing strong poten­tial for future extrac­tion and pro­cess­ing.