How to identify Star Ruby

To correctly identify Star Ruby (Corundum (variety: Ruby, asteriated)), 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

   Various shades of red, from pinkish-red to purplish-red to deep, vivid red. The color is due to trace amounts of chromium.

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 when polished (cabochon cut). Internally, it contains fine, silky inclusions.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Typically found as hexagonal prisms, often tabular. When cut as a Star Ruby, it is always a cabochon to display the asterism.

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 (e.g., marble, gneiss, schist) and igneous rocks (e.g., basalt, syenite). Alluvial deposits are also significant sources, where rubies are weathered out of their primary host rocks and concentrated in riverbeds.

Key Facts

Hardness

9 on the Mohs scale

Specific Gravity

3.97 - 4.05

Crystal System

Trigonal

Color

Red (due to chromium impurities)

Luster

Vitreous to adamantine

Transparency

Opaque to translucent (due to rutile inclusions)

Fracture

Conchoidal to uneven

Cleavage

None (parting present)

Composition

Aluminum oxide (Al2O3) with trace chromium (Cr) for color, and titanium dioxide (TiO2) as rutile inclusions for asterism.

Physical characteristics

• Crystal Habit: Hexagonal prisms, tabular crystals. Asteriated varieties are typically cut as cabochons.
• Cleavage Type: No true cleavage, but exhibits parting along the basal pinacoid {0001} and rhombohedral planes {1011}.
• Fracture Type: Conchoidal to uneven.
• Tenacity: Brittle.
• Luster Type: Vitreous to adamantine.

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