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Tiger's Eye is a captivating gemstone known for its striking chatoyancy, a silky, luminous streak of light that moves across the surface as the stone is rotated. This optical effect, often referred to as the 'cat's eye effect,' is caused by the parallel intergrowth of quartz fibers and altered amphibole minerals, primarily goethite. The color ranges from golden-yellow to reddish-brown, often with dark bands. It is a pseudomorph, meaning it formed when one mineral (crocidolite) was replaced by another (chalcedony/quartz) while retaining the original mineral's fibrous structure.
How to Identify
- Color
- Golden-yellow to reddish-brown, often with dark brown or black bands. Hawk's Eye (blue Tiger's Eye) is blue-gray to blue-green.
- Luster
- Silky to vitreous when polished, exhibiting strong chatoyancy.
- Texture
- Fibrous, often visible on unpolished surfaces or under magnification. Smooth and polished on finished pieces.
- Crystal Form
- Cryptocrystalline, forming fibrous aggregates. Does not exhibit macroscopic crystal forms.
- Cleavage
- None, due to its cryptocrystalline nature.
- Geological Environment
- Typically found in highly metamorphosed iron formations, often associated with asbestos deposits (crocidolite). Occurs in veins or seams within host rocks that have undergone silicification.
Key Facts
- Hardness: 6.5-7 on the Mohs scale
- Specific Gravity: 2.64-2.71
- Crystal System: Trigonal (as quartz/chalcedony)
- Color: Golden-yellow, golden-brown, reddish-brown, sometimes blue-gray (Hawk's Eye)
- Luster: Silky to vitreous, chatoyant
- Transparency: Opaque to translucent
- Fracture: Conchoidal to splintery
- Cleavage: None
- Composition: Silicon dioxide (SiO2) with inclusions of iron oxides (goethite) and relict amphibole fibers.
Quick Check
- Color: Golden-yellow to reddish-brown with dark bands
- Luster: Silky to vitreous, chatoyant
- 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 forms as a pseudomorph of quartz after crocidolite (a fibrous amphibole mineral, specifically riebeckite). The original crocidolite fibers are replaced by chalcedony (cryptocrystalline quartz) while retaining the fibrous structure. Oxidation of the iron within the crocidolite results in the characteristic golden-yellow to reddish-brown colors. This process is known as silicification.
Usage
Primarily used as a gemstone for jewelry (cabochons, beads, carvings), ornamental objects, and decorative items due to its attractive chatoyancy. Historically, it was also believed to have protective and healing properties.
Age Distribution
Typically found in Precambrian to Cenozoic age metamorphic rocks, particularly in areas with serpentinite or amphibolite facies metamorphism.
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 associated with crocidolite asbestos.
India
Smaller deposits are found in various regions.
Myanmar (Burma)
Known for some occurrences.
United States
Limited occurrences in states like California and Arizona.
Canada
Some deposits have been reported.
Finding Tips
Look for Metamorphic Environments
Tiger's Eye is found in metamorphic terrains, particularly those with banded iron formations or areas where crocidolite asbestos was present.
Identify Fibrous Structure
Even in rough form, look for a distinct fibrous texture. The chatoyancy might be less obvious until polished, but the parallel fibers are a key indicator.
Observe Color and Luster
The characteristic golden-yellow to reddish-brown colors and the silky luster (even duller in rough specimens) are good clues.
Check for Pseudomorphs
Understand that you are looking for a silicified replacement of another mineral, so the original fibrous habit will be preserved.
Similar Rocks
Hawk's Eye
Hawk's Eye (a variety of Chalcedony, a cryptocrystalline form of silica, SiO2)
Also known as: Blue Tiger's Eye
Cat's Eye Chrysoberyl
Chrysoberyl (BeAl2O4)
Also known as: Cymophane
Serpentine
Serpentine Group Minerals ((Mg,Fe)3Si2O5(OH)4)
Also known as: Serpentinite
Scientific Classification
- Mineral Class
- Silicate (Tectosilicate)
- Group
- Quartz Group (Chalcedony variety)
- Crystal System
- Trigonal
- Chemical Formula
- SiO2 (with FeO(OH) inclusions)
- Composition
- Silicon dioxide with fibrous inclusions of iron hydroxides (primarily goethite) that replaced original crocidolite fibers.
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