How to identify Ruby

To correctly identify Ruby (Corundum (Al₂O₃) var. Ruby), 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

   Ranges from pinkish-red to deep, vivid 'pigeon's blood' red. The color is due to chromium impurities.

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 on crystal faces, conchoidal fracture surfaces.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Hexagonal prisms, tabular crystals, bipyramidal forms. Often found as irregular grains in alluvial deposits.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   None, but exhibits parting along basal planes and rhombohedral planes due to twinning.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Metamorphic rocks (marble, gneiss, schist), igneous rocks (basalt, syenite, pegmatites), and secondary alluvial deposits.

Key Facts

Hardness

9 (Mohs scale)

Specific Gravity

3.97 - 4.05 g/cm³

Crystal System

Trigonal

Color

Red (due to Cr³⁺ impurities)

Luster

Vitreous to adamantine

Transparency

Transparent to opaque

Fracture

Conchoidal to uneven

Cleavage

None (exhibits parting)

Composition

Aluminum oxide (Al₂O₃) with trace chromium

Physical characteristics

• Crystal Habit: Hexagonal prisms, tabular, bipyramidal; also granular, massive, or in alluvial pebbles.
• Cleavage Type: None, but exhibits parting along {0001} (basal) and {1011} (rhombohedral) planes due to lamellar twinning.
• Fracture Type: Conchoidal to uneven.
• Tenacity: Brittle.
• Luster Type: Vitreous to adamantine.

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