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Material World
Mineral · Al2O3

Corundum

Aluminium oxide — hardness 9, the mineral of ruby and sapphire, and a major industrial abrasive.

Abrasive Gemstone

Corundum is crystalline aluminium oxide, second only to diamond among common natural minerals for hardness. Pure corundum is colourless; trace impurities produce the two most valuable coloured gemstones. Chromium gives ruby its red, and iron with titanium gives sapphire its blue.

The same mineral, in unremarkable brown granular form, is one of the oldest industrial abrasives.

How it forms

Corundum needs aluminium-rich, silica-poor conditions, which is an unusual combination: where silica is abundant, aluminium forms feldspar or mica instead. That restriction is why corundum is not common despite aluminium being the third most abundant element in the crust.

It forms in metamorphosed bauxite and clay-rich rocks, in silica-deficient igneous rocks, and in marbles where impure limestone has recrystallised. Gem-quality material is further restricted by the trace chemistry that produces colour — chromium for ruby, iron and titanium for blue sapphire — and by conditions stable enough for large clear crystals to grow.

Medium confidence Weak evidence

How we know: checked recently · only one source, so there is nothing to cross-check it against · stated directly by the source.

How this connects

Where a connection has been confirmed by an outside reference, that reference is named beside it.

has variety

  • Ruby — mineral variety Wikidata
  • Sapphire — mineral variety

has the same composition as

  • Aluminium oxide — compound · identical composition, entirely different sources and supply chains

contains

is used as

is an alternative to

  • Silicon carbide — compound · as an abrasive; silicon carbide is harder but more friable
  • Diamond — mineral · as a hard synthetic crystal: both are grown industrially, and sapphire is used wherever diamond's hardness is more than the job needs and its price more than the budget
  • Quartz — mineral · as blasting abrasive, and the substitution was made for the operators rather than for the work: silica sand generates respirable dust that causes silicosis, and alumina, garnet and slag replaced it wherever the regulation reached

is a component of

  • Schist — rock · in aluminous schists, and as emery when it occurs with magnetite — the abrasive of the ancient world

is produced by

  • Czochralski process — process · as synthetic sapphire, for watch glasses, LED substrates and transparent armour — grown rather than mined, and identical to the mineral

Sources

  • Material World
    Our own writing
  • Wikidata
    Wikimedia Foundation · Creative Commons CC0 1.0 Universal (public domain dedication)
  • Crystallography Open Database
    COD Advisory Board / Vilnius University · CC0 — contributors place data in the public domain

Questions this page answers

Where it comes from, and what it becomes

Follow Corundum back to what it starts as, and forward into what it becomes. Each step is a documented one — a real route material takes, not a chain of inference.

Upstream — what it comes from

  • Corundum → is produced by (as synthetic sapphire, for watch glasses, LED substrates and transparent armour — grown rather than mined, and identical to the mineral) → Czochralski process → takes as input (charged to the crucible already refined; the process changes its arrangement and its purity, not its identity) → Silicon → is produced by (carbothermic reduction of silica with coke in a submerged arc furnace, which is smelting in the strict sense even though no ore is involved) → Smelting → takes as input (as the element removed, not added) → Oxygen → is produced by (the largest output by tonnage, and the reason air separation units sit beside steelworks) → Air separation
  • Corundum → is produced by (as synthetic sapphire, for watch glasses, LED substrates and transparent armour — grown rather than mined, and identical to the mineral) → Czochralski process → takes as input (charged to the crucible already refined; the process changes its arrangement and its purity, not its identity) → Silicon → is sourced from (reduced with carbon in an electric arc furnace) → Quartz
  • Corundum → is produced by (as synthetic sapphire, for watch glasses, LED substrates and transparent armour — grown rather than mined, and identical to the mineral) → Czochralski process → takes as input (charged to the crucible already refined; the process changes its arrangement and its purity, not its identity) → Silicon → is produced by (carbothermic reduction of silica with coke in a submerged arc furnace, which is smelting in the strict sense even though no ore is involved) → Smelting → takes as input (roasted to drive off sulfur, then reduced to copper) → Chalcopyrite
  • Corundum → is produced by (as synthetic sapphire, for watch glasses, LED substrates and transparent armour — grown rather than mined, and identical to the mineral) → Czochralski process → takes as input (charged to the crucible already refined; the process changes its arrangement and its purity, not its identity) → Silicon → is produced by (carbothermic reduction of silica with coke in a submerged arc furnace, which is smelting in the strict sense even though no ore is involved) → Smelting → takes as input (reduced with carbon to metallic tin) → Cassiterite
  • Corundum → is produced by (as synthetic sapphire, for watch glasses, LED substrates and transparent armour — grown rather than mined, and identical to the mineral) → Czochralski process → takes as input (charged to the crucible already refined; the process changes its arrangement and its purity, not its identity) → Silicon → is produced by (carbothermic reduction of silica with coke in a submerged arc furnace, which is smelting in the strict sense even though no ore is involved) → Smelting → takes as input (reduced with coke in a blast furnace) → Hematite
  • Corundum → is produced by (as synthetic sapphire, for watch glasses, LED substrates and transparent armour — grown rather than mined, and identical to the mineral) → Czochralski process → takes as input (charged to the crucible already refined; the process changes its arrangement and its purity, not its identity) → Silicon → is produced by (carbothermic reduction of silica with coke in a submerged arc furnace, which is smelting in the strict sense even though no ore is involved) → Smelting → takes as input (the phosphate rock charged to the electric furnace, with coke to reduce it and silica to take up the calcium) → Apatite

These are the most distinct paths back. Corundum can be traced through others besides.

Downstream — what it becomes

  • Corundum → is a component of (in aluminous schists, and as emery when it occurs with magnetite — the abrasive of the ancient world) → Schist → is a source for (the next step up in grade, where mica is consumed and the minerals separate into bands) → Gneiss
  • Corundum → is used as → Abrasive