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Tiger's Eye is a captivating variety of chalcedony (microcrystalline quartz) known for its striking chatoyancy, a silky, luminous streak of light that moves across the surface as the stone is rotated. This optical phenomenon is caused by the parallel intergrowth of quartz fibers and altered amphibole minerals, primarily crocidolite, which have been replaced by silica. The characteristic colors range from golden-yellow to rich reddish-brown, often with bands of darker brown or black. The fibrous structure is clearly visible under magnification.
How to Identify
- Color
- Golden-yellow, golden-brown, reddish-brown, sometimes with darker brown or black bands. Hawk's Eye (blue Tiger's Eye) is blue-gray to blue-green.
- Luster
- Silky to vitreous, especially on polished surfaces, with a distinct chatoyancy.
- Texture
- Fibrous, often visible as fine parallel lines or bands. Polished surfaces are smooth.
- Crystal Form
- Massive, cryptocrystalline aggregates, typically forming veins or layers within host rock. Does not form macroscopic crystals.
- Cleavage
- None, as it is a variety of quartz.
- Geological Environment
- Typically found in metamorphic rocks, particularly those rich in iron-bearing amphiboles. It forms in veins or layers within serpentinites or other altered ultramafic rocks.
Key Facts
- Hardness: 7 on the Mohs scale
- Specific Gravity: 2.64 - 2.71
- Crystal System: Trigonal (as quartz)
- Color: Golden-yellow, golden-brown, reddish-brown, blue-gray (Hawk's Eye)
- Luster: Silky to vitreous
- Transparency: Opaque to translucent
- Fracture: Conchoidal to splintery (due to fibrous nature)
- Cleavage: None
- Composition: Silicon dioxide (SiO2) with inclusions of iron oxides and altered amphibole fibers.
Quick Check
- Color: Golden-yellow to reddish-brown with fibrous bands
- Luster: Silky to vitreous, chatoyant
- Streak: White
Physical Characteristics
- Crystal Habit: Massive, fibrous aggregates, pseudomorph after crocidolite.
- Cleavage Type: None (due to quartz composition).
- Fracture Type: Conchoidal to splintery, reflecting the fibrous structure.
- Tenacity: Brittle.
- Luster Type: Silky to vitreous, with strong chatoyancy.
Formation
Tiger's Eye is a pseudomorph of quartz after crocidolite (a fibrous amphibole mineral, specifically riebeckite). The formation process involves the infiltration of silica-rich solutions into rocks containing crocidolite. The silica replaces the crocidolite fibers while retaining their original fibrous structure. Oxidation of the iron within the crocidolite (or the replacing silica) gives Tiger's Eye its characteristic golden-yellow to reddish-brown colors. The parallel alignment of these replaced fibers is responsible for the chatoyancy (cat's eye effect).
Usage
Primarily used as a gemstone for jewelry (cabochons, beads, carvings), ornamental stone, and in lapidary arts. Historically, it was believed to offer protection and bring good fortune.
Age Distribution
Formed in various geological periods, often associated with ancient metamorphic terrains.
Where to Find
South Africa
The primary and most significant source, particularly in the Griqualand West region, where vast deposits are found in banded iron formations.
Australia
Western Australia has notable deposits, often producing material with a slightly different color range.
India
Smaller deposits are found, contributing to the global supply.
Myanmar (Burma)
Known for some occurrences, though less prominent than South Africa.
United States
Limited occurrences in states like Arizona and California.
Finding Tips
Look for Chatoyancy
The most distinctive feature is the moving band of light. Rotate the specimen under a light source to observe this effect.
Examine Color and Pattern
Seek out the characteristic golden-yellow to reddish-brown colors with parallel fibrous bands. Blue-gray varieties are Hawk's Eye.
Check Hardness
Tiger's Eye is quartz, so it should scratch glass (Mohs hardness 7). This helps distinguish it from softer imitations.
Observe Luster
The silky luster is a key indicator, especially on fractured or unpolished surfaces.
Similar Rocks
Hawk's Eye
Quartz (variety)
Also known as: Blue Tiger's Eye
Pietersite
Chalcedony (variety)
Also known as: Tempest Stone
Cat's Eye Chrysoberyl
Chrysoberyl (variety)
Also known as: Cymophane
Scientific Classification
- Mineral Class
- Silicate (Tectosilicate)
- Group
- Quartz Group (Chalcedony variety)
- Crystal System
- Trigonal
- Chemical Formula
- SiO2 (with fibrous inclusions of altered crocidolite)
- Composition
- Silicon dioxide, with iron oxides (goethite, limonite) responsible for color, and relict fibrous structures of amphibole (riebeckite/crocidolite).
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