How to identify Ruby

To correctly identify Ruby (Corundum (Al₂O₃) with Chromium impurities), 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 a deep, vivid 'pigeon's blood' red. The color is often unevenly distributed, appearing as streaks or spots.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Vitreous (glassy) to adamantine (diamond-like) when polished.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Typically smooth on crystal faces, but can be rough on fractured surfaces.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Usually forms as hexagonal prisms with flat basal pinacoids, or as bipyramidal crystals. Can also be found as granular or massive aggregates.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   No true cleavage, but exhibits parting along twinning planes or basal planes, which can be mistaken for cleavage.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Found in metamorphic rocks such as marble, gneiss, and schist, and in some igneous rocks like basalt. Often occurs in alluvial deposits (placers) due to its hardness and resistance to weathering.

Key Facts

Hardness

9 (Mohs scale)

Specific Gravity

3.97 - 4.05

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 (Cr)

Physical characteristics

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

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