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

   Varies from pinkish-red to deep, vivid 'pigeon's blood' red. The most valuable rubies have a pure, vibrant red with a slight purplish secondary hue.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Vitreous (glassy) to adamantine.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Smooth to slightly uneven on fractured surfaces.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Typically forms hexagonal prisms, often with flat basal pinacoid terminations. Can also be found as tabular, bipyramidal, or massive forms. Twinning is common.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

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

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Metamorphic rocks (marbles, gneisses, schists) and certain igneous rocks (alkali basalts, syenites). Often found in alluvial deposits (placers) due to its hardness and resistance to weathering.

Key Facts

Hardness

9 on the Mohs scale

Specific Gravity

3.97 - 4.05 g/cm³

Crystal System

Trigonal

Color

Red, due to chromium impurities

Luster

Vitreous to adamantine

Transparency

Transparent to opaque

Fracture

Conchoidal to uneven

Cleavage

None (exhibits parting)

Composition

Aluminum oxide (Al2O3) with trace chromium (Cr3+)

Physical characteristics

• Crystal Habit: Hexagonal prisms, tabular, bipyramidal, massive. Often twinned.
• Cleavage Type: None, but exhibits parting on basal and rhombohedral planes.
• Fracture Type: Conchoidal to uneven
• Tenacity: Brittle
• Luster Type: Vitreous to adamantine

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