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Tiger's Eye is a captivating variety of chalcedony, a cryptocrystalline form of silica, renowned for its striking chatoyancy, an optical phenomenon resembling the slit pupil of a cat's or tiger's eye. This effect is caused by the parallel intergrowth of quartz fibers and altered amphibole minerals, primarily crocidolite, which have been replaced by silica and stained by iron oxides. The color typically ranges from golden-yellow to reddish-brown, often with iridescent bands that shift with light. It is a pseudomorph, meaning one mineral (quartz) has replaced another (crocidolite) while preserving the original mineral's external form.
How to Identify
- Color
- Golden-yellow to reddish-brown, often with iridescent bands.
- Luster
- Silky to vitreous.
- Texture
- Fibrous appearance, often visible on polished surfaces.
- Crystal Form
- Cryptocrystalline, typically massive with a fibrous structure.
- Cleavage
- None, due to its cryptocrystalline nature.
- Geological Environment
- Typically found in low-grade metamorphic rocks, often associated with serpentinites or other ultramafic rocks where crocidolite forms. It occurs in veins or seams within these host rocks.
Key Facts
- Hardness: 6.5-7 on the Mohs scale.
- Specific Gravity: 2.64-2.71.
- Crystal System: Trigonal (cryptocrystalline aggregates of quartz).
- Color: Golden-yellow, golden-brown, reddish-brown.
- Luster: Silky to vitreous.
- Transparency: Opaque to translucent.
- Fracture: Conchoidal to splintery.
- Cleavage: None.
- Composition: Silicon dioxide (SiO2) with inclusions of iron oxides.
Quick Check
- Color: Golden-yellow to reddish-brown with iridescent bands.
- Luster: Silky to vitreous.
- Streak: White.
Physical Characteristics
- Crystal Habit: Massive, fibrous aggregates.
- Cleavage Type: None.
- Fracture Type: Conchoidal to splintery.
- Tenacity: Brittle.
- Luster Type: Silky (due to fibrous structure) to vitreous (on broken surfaces).
Formation
Tiger's Eye forms as a pseudomorph of quartz after the fibrous mineral crocidolite (a type of asbestos). The original crocidolite fibers are replaced by silica (quartz) while retaining the fibrous structure. The characteristic golden-brown color and chatoyancy are due to the presence of iron oxides (limonite) that stain the quartz fibers, often after the oxidation of the original crocidolite. This process typically occurs in low-grade metamorphic environments.
Usage
Primarily used as a gemstone for jewelry (cabochons, beads, carvings), ornamental objects, and decorative items. Its unique chatoyancy makes it highly sought after.
Age Distribution
Formed in various geological periods, often associated with Precambrian to Cenozoic metamorphic events.
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 California and Arizona.
Finding Tips
Look for Chatoyancy
The most distinctive feature is its chatoyancy. Rotate the specimen under a light source to observe the moving band of light.
Examine Color and Pattern
Observe the characteristic golden-yellow to reddish-brown colors and the parallel fibrous bands.
Check Hardness
Tiger's Eye has a Mohs hardness of 6.5-7, which can be tested with common tools (e.g., it will scratch glass).
Consider Geological Context
If collecting in the field, look for areas with metamorphic rocks, especially those associated with banded iron formations or serpentinites.
Safety Precaution: Crocidolite
While Tiger's Eye itself is quartz and generally safe, its formation involves the replacement of crocidolite (a type of asbestos). Unpolished, raw specimens, especially those with visible blue fibers (Hawk's Eye), might contain residual crocidolite. Avoid inhaling dust from cutting or grinding raw material without proper respiratory protection. Polished Tiger's Eye is considered safe as the asbestos fibers are encapsulated by quartz.
Similar Rocks
Hawk's Eye
Quartz (variety Chalcedony, pseudomorph after Crocidolite)
Also known as: Blue Tiger's Eye
Cat's Eye Chrysoberyl
Chrysoberyl
Also known as: Cymophane
Pietersite
Chalcedony with embedded Crocidolite fibers
Also known as: Tempest Stone
Scientific Classification
- Mineral Class
- Silicates (Tectosilicates)
- Group
- Quartz Group (Chalcedony variety)
- Crystal System
- Trigonal (microcrystalline quartz)
- Chemical Formula
- SiO2 (with FeOOH inclusions)
- Composition
- Silicon dioxide, with iron oxide (limonite) inclusions responsible for color and chatoyancy, formed as a pseudomorph after crocidolite.
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