How to identify Star Ruby

To correctly identify Star Ruby (Corundum (Al2O3) with asterism), 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

   Red to reddish-purple, often with a slightly purplish or brownish tint. The color can range from light pinkish-red to deep, vivid 'pigeon's blood' red.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Vitreous (glassy) to adamantine (diamond-like) on crystal faces; dull to greasy on fracture surfaces.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Typically smooth when polished en cabochon. The star effect is a visual phenomenon, not a tactile texture.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Hexagonal system, typically forming tabular, prismatic, or bipyramidal crystals. Star Rubies are almost exclusively cut en cabochon to display asterism, so natural crystal forms are less commonly seen in finished gems.

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

   Found in metamorphic rocks such as marble, gneiss, and schist, and in igneous rocks like basalt and pegmatite. Due to its hardness and resistance to weathering, it is also commonly found in alluvial (placer) deposits derived from these primary sources.

Key Facts

Hardness

9 on the Mohs scale

Specific Gravity

3.95 - 4.10 g/cm³

Crystal System

Trigonal (Hexagonal division)

Color

Red, reddish-purple, pinkish-red

Luster

Vitreous to adamantine

Transparency

Opaque to translucent (rarely semi-transparent)

Fracture

Conchoidal to uneven

Cleavage

None (exhibits parting)

Composition

Aluminum oxide (Al2O3) with trace chromium (Cr3+) for color, and rutile (TiO2) inclusions for asterism.

Physical characteristics

• Crystal Habit: Typically tabular, prismatic, or bipyramidal in its natural state. Star Rubies are almost exclusively cut en cabochon.
• Cleavage Type: None, but exhibits parting along {0001} (basal) and {1011} (rhombohedral) planes due to lamellar twinning.
• Fracture Type: Conchoidal to uneven, brittle.
• Tenacity: Brittle
• Luster Type: Vitreous to adamantine

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