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Tiger's Eye is a captivating metamorphic rock known for its striking chatoyancy, a silky, luminous streak of light that moves across its surface as it is rotated. This optical phenomenon resembles the eye of a tiger, hence its name. It is a pseudomorph, meaning one mineral (quartz) has replaced another (crocidolite asbestos) while retaining the original mineral's fibrous structure. The characteristic golden-yellow to reddish-brown colors are due to the presence of iron oxides, primarily goethite, which stain the quartz fibers.
How to Identify
- Color
- Golden-yellow, golden-brown, reddish-brown, sometimes blue (Hawk's Eye). The colors are typically banded or streaky.
- Luster
- Silky to vitreous, with a distinct chatoyancy.
- Texture
- Fibrous, often with a fine-grained, compact appearance. Polished surfaces are smooth.
- Crystal Form
- Massive, fibrous aggregates, pseudomorphs after crocidolite. Individual crystals are microscopic.
- Cleavage
- None, due to its quartz composition, but the fibrous structure can create a 'false cleavage' or parting along the fiber direction.
- Geological Environment
- Typically found in metamorphosed banded iron formations, where crocidolite asbestos has been altered by silica-rich hydrothermal fluids.
Key Facts
- Hardness: 6.5-7 on the Mohs scale (due to quartz composition)
- Specific Gravity: 2.64-2.71
- Crystal System: Trigonal (for quartz), but pseudomorphs retain the monoclinic fibrous habit of crocidolite
- Color: Golden-yellow, golden-brown, reddish-brown, blue
- Luster: Silky, vitreous
- Transparency: Opaque to translucent
- Fracture: Conchoidal to splintery (along fiber direction)
- Cleavage: None (quartz), but a parting along the fibrous structure
- Composition: Silicon dioxide (SiO2) with inclusions of iron oxides (goethite) and relict fibrous structure of crocidolite
Quick Check
- Color: Golden-yellow to reddish-brown (oxidized) or blue (unoxidized)
- Luster: Silky to vitreous, with chatoyancy
- Streak: White
Physical Characteristics
- Crystal Habit: Massive, fibrous aggregates, pseudomorphs after crocidolite
- Cleavage Type: None (quartz), but a distinct parting along the fibrous structure
- Fracture Type: Conchoidal to splintery
- Tenacity: Brittle
- Luster Type: Silky to vitreous, with chatoyancy
Formation
Tiger's Eye forms as a pseudomorph of quartz after crocidolite. Crocidolite, a fibrous blue asbestos mineral (a variety of riebeckite), crystallizes first. Subsequently, silica-rich solutions infiltrate the crocidolite, replacing the asbestos fibers while preserving their original fibrous structure. Iron oxides (primarily goethite) then stain the quartz, imparting the characteristic golden-yellow to reddish-brown colors. The chatoyancy (cat's-eye effect) is due to the parallel alignment of the original crocidolite fibers, now replaced by quartz and iron oxides.
Usage
Primarily used as a gemstone for jewelry (cabochons, beads, carvings), ornamental objects, and decorative items. Its unique chatoyancy makes it highly prized.
Age Distribution
Typically found in Precambrian banded iron formations, dating back billions of years.
Where to Find
South Africa
The primary and most significant source, particularly in the Griqualand West region, where it occurs in extensive banded iron formations.
Australia
Western Australia, particularly the Hamersley Range, is another important source.
India
Smaller deposits are found in various regions.
Myanmar (Burma)
Known for some occurrences.
United States
Limited occurrences in Arizona and California.
Finding Tips
Look for Banded Iron Formations
Tiger's Eye is almost exclusively found in ancient banded iron formations, which are sedimentary rocks rich in iron oxides and chert. These formations are typically Precambrian in age.
Identify Fibrous Structures
Even in its raw form, look for a distinct fibrous texture. The chatoyancy might not be as apparent until polished, but the parallel fibers are a key indicator.
Observe Color and Luster
The characteristic golden-brown to reddish-brown colors and the silky luster are good initial identifiers. Unoxidized blue varieties (Hawk's Eye) also occur.
Safety Precautions (Crocidolite Origin)
While the crocidolite asbestos in Tiger's Eye has been completely replaced by quartz, the original formation involved asbestos. When collecting raw, unpolished specimens, especially those that might still contain unreplaced crocidolite or are dusty, exercise extreme caution. Avoid inhaling dust. Wear appropriate respiratory protection (P100 respirator) and eye protection. Wetting the material can help suppress dust. Do not saw, grind, or sand raw material without proper ventilation and personal protective equipment. Polished Tiger's Eye is generally considered safe as the asbestos fibers are encapsulated by quartz.
Similar Rocks
Hawk's Eye
Fibrous Crocidolite (unoxidized, a variety of Quartz)
Also known as: Blue Tiger's Eye
Pietersite
Brecciated Tiger's Eye and Hawk's Eye
Also known as: Tempest Stone
Cat's Eye Chrysoberyl
Chrysoberyl (BeAl2O4)
Also known as: Cymophane
Scientific Classification
- Mineral Class
- Silicate (Tectosilicate)
- Group
- Quartz Group
- Crystal System
- Trigonal (for quartz)
- Chemical Formula
- SiO2 (with FeO(OH) inclusions)
- Composition
- Silicon dioxide, with iron oxide impurities responsible for color and the fibrous structure inherited from crocidolite
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