Corundum (Al₂O₃) is the parent species of two of the most prized gem minerals on Earth — ruby (red, chromium-colored) and sapphire (every other color: blue, pink, yellow, green, padparadscha). It is the second-hardest natural mineral after diamond, with a Mohs hardness of 9, and crystallizes in the trigonal system as hexagonal prisms, barrels, and tabular crystals. Mogok (Myanmar) and Sri Lanka are the historical world standards; Yunnan hosts modest Chinese deposits.
Key Facts
- Mohs hardness 9 — second-hardest mineral, defining the Mohs scale point.
- Trigonal symmetry; hexagonal prismatic, tabular, and barrel-shaped crystal habits.
- Color is determined by trace impurities: chromium → ruby, iron+titanium → blue sapphire, vanadium → purple, iron alone → yellow/green.
- Used industrially as abrasive (emery) when fine-grained.
- Star ruby and star sapphire show six-rayed asterism from rutile silk inclusions.
Notable Localities
Mogok (Myanmar) is the historical world standard for ruby. Ratnapura (Sri Lanka) and Ilakaka (Madagascar) yield gem sapphires. Yunnan hosts Chinese deposits with metamorphic blue-gray crystals.
Found at these Localities
- Mogok Stone Tract (抹谷宝石产地)
- Yunnan (云南)
