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Star Ruby is a rare and valuable variety of ruby that exhibits a phenomenon called asterism, typically a six-rayed star. This optical effect is caused by reflections from microscopic, needle-like inclusions of rutile (titanum dioxide) that are oriented in three directions at 60-degree angles within the crystal structure. The star is best observed under a single point of light and when the stone is cut into a smooth, domed cabochon. The color ranges from pinkish-red to purplish-red to a deep, vivid red, often referred to as 'pigeon's blood' red, which is the most prized hue.
How to Identify
- Color
- Various shades of red, from pinkish-red to purplish-red to deep, vivid red. The color is due to trace amounts of chromium.
- Luster
- Vitreous (glassy) to adamantine.
- Texture
- Smooth when polished (cabochon cut). Internally, it contains fine, silky inclusions.
- Crystal Form
- Typically found as hexagonal prisms, often tabular. When cut as a Star Ruby, it is always a cabochon to display the asterism.
- Cleavage
- None, but exhibits parting along basal planes and rhombohedral planes due to lamellar twinning.
- Geological Environment
- 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.
Quick Check
- Color: Red (various shades)
- Luster: Vitreous to adamantine
- Streak: White
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.
Formation
Star Rubies form under high-temperature and high-pressure metamorphic or igneous conditions. The asterism (star effect) is caused by the presence of precisely oriented, needle-like inclusions of rutile (titanium dioxide, TiO2). These inclusions, known as 'silk', reflect light in a six-rayed star pattern when the stone is cut en cabochon. The rutile needles exsolve from the corundum during cooling, aligning themselves along the crystallographic axes of the ruby.
Usage
Primarily used as a gemstone in jewelry due to its unique asterism and vibrant red color. Highly prized by collectors.
Age Distribution
Precambrian to Cenozoic, depending on the host rock formation.
Where to Find
Myanmar (Burma)
Historically, the Mogok Valley in Myanmar has been the most famous source of high-quality rubies, including Star Rubies, known for their 'pigeon's blood' color.
Sri Lanka
Known for producing a wide range of corundum, including Star Rubies, often with a lighter red or pinkish-red hue.
Thailand
Chanthaburi and Trat provinces are known for their ruby deposits, some of which yield asteriated varieties.
Madagascar
Newer deposits have yielded significant quantities of rubies, including some with asterism.
Tanzania
The Winza and Longido regions are known for ruby production, with some asteriated specimens.
Vietnam
Luc Yen and Quy Chau districts are important sources of rubies, including star varieties.
Finding Tips
Look for Alluvial Deposits
Many rubies, including star rubies, are found in secondary alluvial deposits. Look for gravel beds in ancient or modern river systems, especially in regions known for corundum.
Examine Host Rocks
In primary deposits, rubies are found in marble (metamorphosed limestone), gneiss, schist, or basalt. Look for these rock types in known ruby-producing areas.
Check for Asterism
When examining rough or cabochon-cut material, use a single point of light (like a penlight) to check for the characteristic star effect. The star should be sharp and centered.
Consult Geological Maps
Utilize geological maps and reports for known corundum occurrences to narrow down potential prospecting areas.
Safety Precautions
Always obtain proper permits for prospecting. Be aware of local wildlife and environmental conditions. When working in stream beds, be mindful of water levels and currents. No specific chemical hazards are associated with handling Star Ruby itself, but always practice general safety in the field.
Similar Rocks
Star Sapphire
Corundum (variety: Sapphire, asteriated)
Also known as: Asteriated Sapphire
Star Garnet
Almandine Garnet (asteriated)
Also known as: Asteriated Garnet
Star Diopside
Diopside (asteriated)
Also known as: Black Star Diopside
Scientific Classification
- Mineral Class
- Oxides
- Group
- Corundum Group
- Crystal System
- Trigonal
- Chemical Formula
- Al2O3 (with trace Cr and TiO2 inclusions)
- Composition
- Aluminum oxide, with chromium responsible for the red color and oriented rutile needles (titanium dioxide) causing the asterism.
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