How to identify Corundum

To correctly identify Corundum (Al₂O₃), check each of these features in order. Many rocks and minerals look alike, so cross-reference multiple properties before deciding.

Step-by-step identification

1. 1

   Color

   Overall color and any zoning or banding

   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.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Vitreous to adamantine.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Typically smooth to slightly rough on crystal faces; granular in massive forms.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Hexagonal dipyramidal, tabular, prismatic, or barrel-shaped crystals. Often exhibits striations perpendicular to the c-axis. Can also be massive or granular.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   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.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   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)

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.

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