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Tiger's Eye is a captivating variety of chalcedony, characterized by its silky luster and striking chatoyancy, which resembles the eye of a tiger. It typically exhibits golden-yellow to reddish-brown bands, often with iridescent flashes. This optical phenomenon is caused by the reflection of light off parallel fibrous inclusions, which are pseudomorphs of quartz after crocidolite asbestos, stained by iron oxides. It is a durable and relatively common gemstone.
How to Identify
- Color
- Golden-yellow, golden-brown, reddish-brown, sometimes with darker brown or black bands. The color is due to iron oxide staining.
- Luster
- Silky to vitreous, especially noticeable on polished surfaces due to chatoyancy.
- Texture
- Fine-grained, fibrous appearance internally, often smooth and polished externally for commercial use.
- Crystal Form
- Cryptocrystalline (microcrystalline) aggregates, typically massive or in veins. Does not form macroscopic crystals.
- Cleavage
- None, due to its microcrystalline nature.
- Geological Environment
- Found in metamorphic rocks, particularly those rich in iron and silica, where crocidolite asbestos has been altered. Often occurs in veins or seams within serpentinites or other mafic/ultramafic metamorphic 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
- Transparency: Opaque to translucent
- Fracture: Conchoidal to splintery
- Cleavage: None
- Composition: Silicon dioxide (SiO2) with fibrous inclusions of goethite (FeO(OH))
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 forms through a pseudomorphic replacement process. Initially, fibrous crocidolite (a blue asbestos mineral, Na2(Fe2+,Mg)3Fe3+2Si8O22(OH)2) crystallizes within veins in metamorphic rocks. Over time, silica-rich hydrothermal fluids infiltrate these veins. The silica (SiO2) then replaces the crocidolite fibers, molecule by molecule, while preserving the original fibrous structure. Iron oxides (primarily goethite, FeO(OH)) subsequently oxidize the iron within the replaced crocidolite, imparting the characteristic golden-yellow to reddish-brown colors. The chatoyancy (cat's-eye effect) is a result of light reflecting off the parallel fibrous inclusions of goethite within the microcrystalline quartz.
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
Typically found in Precambrian to Cenozoic age metamorphic rocks, depending on the host rock formation.
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 banded iron formations.
India
Smaller deposits are found in various regions.
Myanmar (Burma)
Known for some occurrences.
United States
Limited occurrences in states like Arizona and California.
Canada
Some deposits have been reported.
Finding Tips
Look for Metamorphic Terrains
Focus on areas known for metamorphic rocks, especially those with banded iron formations or serpentinite bodies, as these are the typical host rocks.
Identify Veins and Seams
Tiger's Eye often occurs in distinct veins or seams within the host rock. Look for linear features or concentrations of fibrous material.
Observe Chatoyancy
The most distinctive feature is its chatoyancy. Rotate suspected samples under a light source to observe the moving band of reflected light.
Check for Color and Luster
The characteristic golden-yellow to reddish-brown colors and silky luster are strong indicators.
Safety Precaution: Crocidolite
While Tiger's Eye itself is safe due to the complete replacement of crocidolite by quartz, the original formation involves crocidolite asbestos. When collecting in situ, especially from raw, unaltered veins, there is a potential risk of encountering unreplaced crocidolite. Crocidolite is a hazardous amphibole asbestos mineral. Avoid disturbing or collecting any blue, fibrous material that appears unaltered. Always wear appropriate respiratory protection (P100 respirator) and avoid creating dust when working with raw material from such environments. Handle with gloves and wash hands thoroughly after contact. Polished Tiger's Eye is completely safe as the asbestos has been fully silicified.
Similar Rocks
Hawk's Eye
Hawk's Eye (a variety of Chalcedony, which is microcrystalline Quartz)
Also known as: Blue Tiger's Eye
Pietersite
Pietersite (a brecciated chalcedony containing hawk's eye and tiger'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 (Chalcedony variety)
- Crystal System
- Trigonal
- Chemical Formula
- SiO2 (with FeO(OH) inclusions)
- Composition
- Silicon dioxide with iron hydroxide inclusions
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