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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 is caused by the parallel growth of quartz fibers and altered amphibole fibers, primarily limonite, which imparts its characteristic golden-brown to reddish-brown colors. It is a pseudomorph, meaning it formed when quartz replaced another mineral (crocidolite asbestos) while preserving its original fibrous structure. The degree of chatoyancy and color intensity can vary, making each piece unique.
How to Identify
- Color
- Golden-brown, yellow-brown, reddish-brown, sometimes blue (Hawk's Eye). The colors are typically banded or streaked.
- Luster
- Silky to vitreous, especially on polished surfaces, due to chatoyancy.
- Texture
- Fibrous internal structure, often visible as parallel lines or bands. Smooth to the touch when polished.
- Crystal Form
- Massive, fibrous aggregates. Does not form macroscopic crystals.
- Cleavage
- None, as it is cryptocrystalline quartz.
- Geological Environment
- Typically found in metamorphosed ultramafic rocks, often associated with serpentinite bodies, where crocidolite asbestos veins have been replaced by quartz. It forms 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)
- Color: Golden-brown, yellow-brown, reddish-brown, blue (Hawk's Eye)
- Luster: Silky to vitreous
- Transparency: Opaque to translucent
- Fracture: Conchoidal to splintery
- Cleavage: None
- Composition: Silicon dioxide (SiO2) with inclusions of limonite and relict crocidolite fibers
Quick Check
- Color: Golden-brown to reddish-brown with parallel bands
- Luster: Silky to vitreous, exhibiting strong chatoyancy
- Streak: White
Physical Characteristics
- Crystal Habit: Massive, fibrous aggregates, pseudomorph after crocidolite
- Cleavage Type: None
- Fracture Type: Conchoidal to splintery
- Tenacity: Brittle
- Luster Type: Silky (chatoyant) to vitreous
Formation
Tiger's Eye is a pseudomorph of quartz after crocidolite asbestos. It forms when quartz (SiO2) slowly replaces the fibrous amphibole mineral crocidolite, while retaining the original fibrous structure. The characteristic chatoyancy (cat's-eye effect) is due to the parallel intergrowth of quartz fibers and altered amphibole fibers (primarily limonite, an iron hydroxide, which gives it its golden-brown color). This replacement process occurs under specific hydrothermal conditions, often in serpentinite or other ultramafic rocks that have undergone metamorphism.
Usage
Primarily used as an ornamental stone, in jewelry (cabochons, beads), carvings, and decorative objects. Its unique chatoyancy makes it highly prized. Historically, it was believed to offer protection and bring good luck.
Age Distribution
Formed in various geological periods, often associated with Precambrian to Cenozoic metamorphic events where crocidolite asbestos was present.
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 found in Western Australia, often exhibiting a more reddish hue.
India
Smaller deposits with varying quality.
Myanmar (Burma)
Known for some occurrences.
United States
Limited occurrences in states like Arizona and California.
Canada
Some minor deposits.
Finding Tips
Look for Chatoyancy
The most distinctive feature is the moving band of light. Rotate the specimen under a light source to observe this effect. Unpolished pieces may show a duller luster but the fibrous structure should still be discernible.
Examine Color and Banding
Tiger's Eye typically exhibits golden-brown, yellow, or reddish-brown colors, often in parallel bands or streaks. Blue varieties (Hawk's Eye) are also common.
Check Hardness
As a variety of quartz, it will scratch glass and steel (Mohs hardness 6.5-7). This helps distinguish it from softer imitations.
Consider Geological Context
If collecting in the field, look for areas known for metamorphosed ultramafic rocks or banded iron formations, which are the typical host rocks for Tiger's Eye.
Beware of Imitations
Glass or plastic imitations may lack the natural fibrous structure and the distinct, sharp chatoyancy of genuine Tiger's Eye. Dyed chalcedony or fiber optic glass can also mimic its appearance.
Similar Rocks
Hawk's Eye
Hawk's Eye (a variety of Chalcedony, which is cryptocrystalline Quartz)
Also known as: Blue Tiger's Eye
Cat's Eye Chrysoberyl
Chrysoberyl (BeAl2O4)
Also known as: Cymophane
Serpentine
Serpentine Group Minerals (e.g., Antigorite, Lizardite, Chrysotile)
Also known as: Serpentinite
Scientific Classification
- Mineral Class
- Silicates (Tectosilicates)
- Group
- Quartz Group (Chalcedony variety)
- Crystal System
- Trigonal (cryptocrystalline)
- Chemical Formula
- SiO2 (with FeO(OH) inclusions)
- Composition
- Silicon dioxide with fibrous inclusions of iron hydroxides (limonite) and relict amphibole (crocidolite)
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