Corundum is a very hard mineral made of crystalline aluminium oxide (chemical formula Al2O3\text{Al}_2\text{O}_3Al2​O3​).

What is corundum?

  • It is a rock-forming mineral composed mainly of aluminium oxide with tiny amounts of other metals like iron, titanium, and chromium.
  • It is naturally transparent, but impurities give it many colors (red, blue, pink, yellow, green, colorless, etc.).
  • On the Mohs hardness scale, corundum has hardness 9, making it one of the hardest common minerals, just below diamond.

In simple terms: corundum is the mineral family that includes rubies and sapphires, and it is also widely used as an industrial abrasive because it is so hard.

Gemstone side: ruby and sapphire

  • Red corundum = ruby , colored mainly by chromium.
  • Blue corundum = blue sapphire , typically colored by iron and titanium.
  • Any other color (yellow, pink, green, etc.) is usually called “fancy sapphire.”

These gemstones are prized because they combine beauty, high durability, and rarity, so they are common in high‑end jewelry and royal collections.

Physical properties (quick facts)

  • Chemical formula: Al2O3\text{Al}_2\text{O}_3Al2​O3​.
  • Crystal system: trigonal; often forms prismatic or tabular crystals.
  • Hardness: 9 on Mohs scale.
  • Streak: colorless/white (harder than the streak plate).
  • Specific gravity: about 3.9–4.1, relatively dense for a non‑metallic mineral.

These features make corundum easy to distinguish from many other transparent minerals in gemology and geology.

Where it forms and how it’s used

  • Formation: found in igneous and metamorphic rocks, and also as loose grains in river gravels (alluvial deposits) because it resists weathering.
  • Main uses:
    • Gemstones (ruby and sapphire in jewelry).
* Industrial abrasives (grinding wheels, sandpapers, cutting tools) thanks to its hardness.

* Refractory and specialized industrial materials where high heat and wear resistance are needed.

Example: the “emery” used in some sandpapers and grinding applications is largely made of corundum mixed with other minerals.

Mini FAQ: quick scoop style

  • Is corundum rare?
    As a mineral, corundum is not extremely rare, but fine gem‑quality ruby and sapphire crystals are rare and valuable.
  • Is all sapphire corundum?
    Yes. By definition, sapphire is any gem‑quality corundum that is not red (which would instead be called ruby).
  • Why is corundum important today?
    It remains central in jewelry trends (colored gemstone engagement rings, fancy sapphires) and is still widely used in abrasives and high‑tech components due to its durability.

Information gathered from public forums or data available on the internet and portrayed here.