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Tiger's Eye is a captivating variety of chalcedony (cryptocrystalline quartz) known for its striking chatoyancy, a silky, luminous streak of light that moves across the surface as the stone is rotated. This optical effect, reminiscent of a cat's or tiger's eye, is caused by the parallel intergrowth of quartz fibers and altered amphibole minerals (originally crocidolite) that have been replaced by silica and stained by iron oxides. The color ranges from golden-yellow to reddish-brown, often with bands of darker brown or black.
How to Identify
- Color
- Golden-yellow, golden-brown, reddish-brown, sometimes with darker brown or black bands. The color is due to iron oxide inclusions.
- Luster
- Silky to vitreous, especially on polished surfaces, exhibiting strong chatoyancy.
- Texture
- Fibrous, often visible on broken surfaces, but typically smooth and polished for commercial use.
- Crystal Form
- Cryptocrystalline, meaning individual crystals are too small to be seen without high magnification. Forms as fibrous aggregates.
- Cleavage
- None, due to its cryptocrystalline nature and fibrous structure.
- Geological Environment
- Typically found in highly metamorphosed iron formations, where crocidolite asbestos has been altered and replaced by silica. Often occurs in veins or layers within these host rocks.
Key Facts
- Hardness: 6.5-7 on the Mohs scale
- Specific Gravity: 2.64-2.71 g/cm³
- Crystal System: Trigonal (as quartz)
- Color: Golden-yellow, golden-brown, reddish-brown
- Luster: Silky to vitreous, chatoyant
- Transparency: Opaque to translucent
- Fracture: Conchoidal to splintery
- Cleavage: None
- Composition: Silicon dioxide (SiO₂) with fibrous inclusions of altered crocidolite and iron oxides
Quick Check
- Color: Golden-yellow to reddish-brown with chatoyancy
- Luster: Silky to vitreous
- Streak: White
Physical Characteristics
- Crystal Habit: Cryptocrystalline, fibrous aggregates, pseudomorph after crocidolite
- Cleavage Type: None
- Fracture Type: Conchoidal to splintery
- Tenacity: Brittle
- Luster Type: Silky to vitreous
Formation
Tiger's Eye is a pseudomorph of quartz after crocidolite (a fibrous blue asbestos mineral). The formation process involves the infiltration of silica-rich solutions into crocidolite, where the silica replaces the crocidolite fibers while retaining their original fibrous structure. Iron oxides (primarily limonite) then stain the silica, giving Tiger's Eye its characteristic golden-yellow to reddish-brown color and chatoyancy. This process is a form of metasomatism, specifically silicification.
Usage
Primarily used as an ornamental stone, in jewelry (cabochons, beads), carvings, and decorative objects. Its chatoyancy makes it highly prized. Historically, it was believed to offer protection and good fortune.
Age Distribution
Formation typically occurs in ancient metamorphic terrains, often associated with Precambrian to Paleozoic age rocks, though the pseudomorphic replacement process can occur over geological timescales.
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
Deposits are found in Western Australia, often associated with iron ore deposits.
India
Smaller occurrences have been noted.
Myanmar (Burma)
Known for some deposits.
United States
Limited occurrences in states like Arizona and California.
Finding Tips
Look for Metamorphic Terrains
Tiger's Eye is found in regions with ancient metamorphic rocks, particularly banded iron formations. Research geological maps of such areas.
Identify Associated Minerals
It often occurs with other iron-rich minerals and sometimes remnants of the original crocidolite (Hawk's Eye). Be aware of the potential presence of asbestos (crocidolite) in its unaltered form.
Observe Chatoyancy
The most distinctive feature is its chatoyancy. Look for the moving band of light when rotating a specimen under a light source. This is best observed on fractured or polished surfaces.
Check for Fibrous Structure
On unpolished or broken surfaces, a fine fibrous texture might be discernible, indicating its pseudomorphic origin.
Similar Rocks
Hawk's Eye
Hawk's Eye (variety of Chalcedony/Quartz)
Also known as: Blue Tiger's Eye
Pietersite
Pietersite (brecciated Tiger's Eye and Hawk's Eye)
Also known as: Tempest Stone
Serpentine
Serpentine Group Minerals
Also known as: New Jade
Scientific Classification
- Mineral Class
- Silicates (Tectosilicates)
- Group
- Quartz Group (Chalcedony variety)
- Crystal System
- Trigonal
- Chemical Formula
- SiO₂ (with inclusions)
- Composition
- Silicon dioxide with fibrous inclusions of altered crocidolite (magnesio-riebeckite) and iron oxides (limonite).
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