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Corundum is a crystalline form of aluminum oxide (Al₂O₃) and is one of the hardest naturally occurring minerals, second only to diamond. Its exceptional hardness makes it highly resistant to scratching and abrasion. Pure corundum is colorless, but trace amounts of impurities give rise to a wide range of colors, leading to its famous gem varieties: ruby (red, due to chromium impurities) and sapphire (all other colors, including blue, pink, yellow, green, purple, orange, and colorless, due to iron, titanium, and vanadium impurities). It typically forms hexagonal prismatic crystals, often with pyramidal terminations or barrel-shaped forms. It lacks true cleavage but exhibits parting along specific crystallographic planes.
How to Identify
- Color
- Highly variable; colorless, white, gray, blue, red, pink, yellow, green, purple, orange, brown, black. Ruby is red, Sapphire is typically blue but can be any other color.
- Luster
- Vitreous to adamantine.
- Texture
- Typically smooth to slightly rough on crystal faces; granular in massive forms.
- Crystal Form
- Hexagonal dipyramidal, tabular, prismatic, or barrel-shaped crystals. Often exhibits striations perpendicular to the c-axis. Can also be massive or granular.
- Cleavage
- No true cleavage, but exhibits parting (pseudocleavage) parallel to the basal pinacoid {0001} and rhombohedral planes {1011}. This parting is due to lamellar twinning or exsolution lamellae.
- Geological Environment
- Metamorphic rocks (schists, gneisses, marbles, emeryites), silica-deficient igneous rocks (nepheline syenites, alkali basalts, lamprophyres), desilicated pegmatites, and placer deposits derived from these primary sources.
Key Facts
- Hardness: 9 (Mohs scale)
- Specific Gravity: 3.95 - 4.10 g/cm³ (typically 4.0)
- Crystal System: Trigonal
- Color: Colorless, white, gray, blue, red, pink, yellow, green, purple, orange, brown, black
- Luster: Vitreous to adamantine
- Transparency: Transparent to opaque
- Fracture: Conchoidal to uneven
- Cleavage: None (exhibits parting)
- Composition: Aluminum oxide (Al₂O₃)
Quick Check
- Color: Highly variable (red, blue, yellow, green, colorless, etc.)
- Luster: Vitreous to adamantine
- Streak: White
Physical Characteristics
- Crystal Habit: Hexagonal dipyramidal, tabular, prismatic, barrel-shaped; also massive, granular, or lamellar.
- Cleavage Type: None; exhibits parting on {0001} and {1011}.
- Fracture Type: Conchoidal to uneven.
- Tenacity: Brittle.
- Luster Type: Vitreous to adamantine.
Formation
Corundum (Al₂O₃) is an aluminum oxide mineral that forms under high-temperature and/or high-pressure conditions. It typically crystallizes from magmas that are undersaturated in silica (e.g., nepheline syenites, alkali basalts) or in metamorphic rocks (e.g., schists, gneisses, marbles) where aluminum is abundant and silica is deficient. It can also form in contact metamorphic aureoles around igneous intrusions, or as a detrital mineral in placer deposits due to its extreme hardness and resistance to weathering.
Usage
Gemstone (ruby, sapphire), abrasive (due to its hardness, used in sandpaper, grinding wheels, polishing compounds), watch bearings, scientific instrument components, laser technology (synthetic ruby and sapphire).
Age Distribution
Corundum forms in a wide range of geological ages, from Precambrian to Cenozoic, depending on the specific geological environment. It is often found in ancient metamorphic and igneous rocks.
Where to Find
Myanmar (Burma)
Famous for high-quality rubies (Mogok Valley) and sapphires.
Sri Lanka
Known for a wide variety of sapphires (blue, pink, yellow, padparadscha) and rubies, often found in alluvial deposits.
Thailand
Historically significant for rubies and blue sapphires, particularly from Chanthaburi and Trat provinces.
Cambodia
Produces rubies and sapphires, often associated with basaltic rocks.
Madagascar
A major source of sapphires (blue, pink, yellow, green) and rubies, found in various geological settings.
Australia
Significant producer of blue and yellow sapphires, especially from Queensland and New South Wales.
United States
Localities in Montana (Yogo Gulch for blue sapphires, Rock Creek for various colors) and North Carolina (rubies and sapphires in metamorphic rocks).
India
Historical source of rubies and sapphires, particularly from Kashmir (famous for 'cornflower blue' sapphires).
Tanzania
Produces rubies (e.g., Winza, Longido) and sapphires (e.g., Tunduru, Umba Valley).
Kenya
Known for rubies and sapphires.
Finding Tips
Look in Placer Deposits
Due to its high specific gravity and hardness, corundum often concentrates in alluvial (placer) deposits. Look for gravels in stream beds, ancient river terraces, and weathered eluvial deposits derived from primary sources.
Identify Associated Minerals
In primary metamorphic settings, corundum can be found with minerals like kyanite, sillimanite, andalusite, garnet, spinel, mica, and feldspar. In igneous settings, it may be associated with nepheline, feldspathoids, and amphiboles.
Check for Hexagonal Crystal Forms
Corundum often forms distinctive hexagonal prismatic or barrel-shaped crystals. Look for these characteristic shapes, even in weathered or broken fragments.
Test Hardness
Corundum is extremely hard (9 on Mohs scale). It will scratch almost all other common minerals, including quartz. This is a key diagnostic property, but be cautious not to damage valuable specimens.
Examine for Parting
While not true cleavage, corundum often exhibits parting. Look for flat, parallel planes of weakness within the crystal, which can sometimes be mistaken for cleavage.
Consider Geological Context
Focus your search in regions known for silica-deficient igneous rocks or high-grade metamorphic terrains, especially those with aluminum-rich protoliths.
Similar Rocks
Spinel
MgAl₂O₄
Also known as: Magnesium Aluminum Oxide
Garnet
X₃Y₂(SiO₄)₃
Also known as: Various species (e.g., Almandine, Pyrope)
Topaz
Al₂SiO₄(F,OH)₂
Also known as: Aluminum Fluorine Silicate
Beryl
Be₃Al₂Si₆O₁₈
Also known as: Emerald, Aquamarine
Scientific Classification
- Mineral Class
- Oxides
- Group
- Corundum Group
- Crystal System
- Trigonal
- Chemical Formula
- Al₂O₃
- Composition
- Aluminum oxide
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