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

   Adamantine to vitreous.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Typically crystalline, often with a somewhat greasy feel in massive forms.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Hexagonal system, commonly forming barrel-shaped, tabular, or pyramidal crystals. Striations parallel to the basal pinacoid are common.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   No true cleavage, but exhibits parting (pseudocleavage) parallel to the basal pinacoid and rhombohedral faces due to lamellar twinning.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Metamorphic rocks (schists, gneisses, marbles), igneous rocks (syenites, nepheline syenites, pegmatites), desilicated environments, and placer deposits.

Key Facts

Hardness

9 (Mohs scale)

Specific Gravity

3.95 - 4.10 g/cm³

Crystal System

Trigonal (often described as Hexagonal in older texts due to its hexagonal appearance)

Color

Highly variable: colorless, white, gray, blue, red, pink, yellow, green, purple, orange, brown, black

Luster

Adamantine to vitreous

Transparency

Transparent to opaque

Fracture

Conchoidal to uneven

Cleavage

None (exhibits parting)

Composition

Aluminum oxide (Al2O3)

Physical characteristics

• Crystal Habit: Hexagonal prisms, often barrel-shaped, tabular, or pyramidal. May be massive or granular.
• Cleavage Type: None, but exhibits parting on {0001} (basal) and {1011} (rhombohedral) planes due to polysynthetic twinning.
• Fracture Type: Conchoidal to uneven
• Tenacity: Brittle
• Luster Type: Adamantine to vitreous

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