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Star Ruby is a rare and valuable variety of the mineral Corundum (aluminum oxide, Al2O3) that exhibits a phenomenon known as asterism. This optical effect manifests as a six-rayed star (or rarely, a twelve-rayed star) that appears to glide across the surface of the stone when illuminated by a single light source. The ruby's characteristic red color is due to trace amounts of chromium (Cr3+) substituting for aluminum in the crystal lattice. The asterism is caused by precisely oriented microscopic needle-like inclusions, typically rutile (titanium dioxide), which reflect light along specific crystallographic directions. These inclusions are often so fine that they are not visible to the naked eye without magnification. Star Rubies are typically cut en cabochon to best display the star effect.
How to Identify
- Color
- 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.
- Luster
- Vitreous (glassy) to adamantine (diamond-like) on crystal faces; dull to greasy on fracture surfaces.
- Texture
- Typically smooth when polished en cabochon. The star effect is a visual phenomenon, not a tactile texture.
- Crystal Form
- 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.
- Cleavage
- None, but exhibits parting along basal planes and rhombohedral planes due to lamellar twinning.
- Geological Environment
- 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.
Quick Check
- Color: Red to reddish-purple
- Luster: Vitreous to adamantine
- Streak: White
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
Formation
Star Rubies form under high-temperature and high-pressure metamorphic or igneous conditions. The asterism (star effect) is caused by the presence of oriented microscopic needle-like inclusions, typically rutile (TiO2), which reflect light in a six-rayed or, less commonly, twelve-rayed star pattern. These inclusions align along the crystallographic axes of the corundum during its growth.
Usage
Primarily used as a gemstone in jewelry due to its rarity, beauty, and the unique asterism phenomenon. High-quality Star Rubies are highly prized and valuable.
Age Distribution
Precambrian to Cenozoic, depending on the host rock formation.
Where to Find
Myanmar (Burma)
Historically significant source, particularly the Mogok Stone Tract, known for producing high-quality rubies, including star rubies, from marble deposits.
Sri Lanka (Ceylon)
A classic source for rubies and star rubies, primarily from alluvial deposits. Sri Lankan star rubies often exhibit a distinct purplish-red hue.
Thailand
Chanthaburi and Trat provinces were significant sources, often producing rubies with a brownish or purplish tint, some of which exhibit asterism.
Vietnam
Luc Yen and Quy Chau districts are known for producing rubies, including star rubies, from marble-hosted deposits.
Madagascar
Newer deposits have yielded significant quantities of rubies and star rubies, often from metamorphic terrains.
Tanzania
The Winza and Longido areas have produced rubies, some with asterism, from metamorphic rocks.
India
Various localities, particularly in Odisha and Karnataka, have produced rubies, including some star rubies, from metamorphic rocks and alluvial deposits.
Finding Tips
Look for Cabochon Cuts
Star Rubies are almost always cut en cabochon (domed, unfaceted) to properly display the asterism. A faceted star ruby would not show the star effect.
Single Light Source Test
To observe the star, illuminate the stone with a single, strong point light source (like a penlight or direct sunlight). The star should appear to move across the dome as the light source or stone is rotated.
Examine Inclusions
Under magnification, the fine, needle-like rutile inclusions responsible for the asterism may be visible, often oriented in three sets at 60-degree angles to each other.
Check for Natural vs. Synthetic
Natural star rubies often have natural inclusions, color zoning, and growth patterns. Synthetic star rubies (e.g., Verneuil process) typically show curved growth lines and gas bubbles, and their stars can appear too perfect or 'painted on'.
Assess Star Quality
A high-quality star is sharp, centered, complete (all six rays visible), and moves smoothly across the dome. The intensity of the star should be strong against the body color of the ruby.
Similar Rocks
Star Sapphire
Corundum (Al2O3) with asterism
Also known as: Asteriated Sapphire
Star Garnet
Almandine or Almandine-Pyrope Garnet with asterism
Also known as: Asteriated Garnet
Cat's Eye Chrysoberyl
Chrysoberyl (BeAl2O4) with chatoyancy
Also known as: Cymophane
Scientific Classification
- Mineral Class
- Oxides
- Group
- Corundum Group
- Crystal System
- Trigonal (Hexagonal division)
- Chemical Formula
- Al2O3
- Composition
- Aluminum oxide. The red color is due to trace amounts of chromium (Cr3+). The asterism is caused by oriented needle-like inclusions, primarily rutile (TiO2).
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