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Tiger's Eye is a captivating chatoyant gemstone, typically golden to reddish-brown, characterized by a silky luster and a luminous band of light that moves across its surface as it is rotated. This optical effect, known as chatoyancy (or 'cat's eye effect'), is caused by the reflection of light off the parallel fibrous inclusions of altered crocidolite within the quartz. It is a pseudomorph of quartz after crocidolite, meaning quartz has replaced the original crocidolite fibers while retaining their fibrous structure.
How to Identify
- Color
- Golden-yellow, golden-brown, reddish-brown, sometimes blue (Hawk's Eye). The color is due to iron oxides.
- Luster
- Silky to vitreous, exhibiting strong chatoyancy.
- Texture
- Fibrous, often with a fine-grained, compact appearance. When polished, it feels smooth.
- Crystal Form
- Massive, fibrous aggregates. Does not form macroscopic crystals of its own, as it is a pseudomorph.
- Cleavage
- None, due to its microcrystalline quartz nature. The fibrous structure of the original crocidolite is preserved but not true cleavage.
- Geological Environment
- Typically found in metamorphosed banded iron formations (BIFs) or other iron-rich sedimentary rocks that have undergone low-grade regional metamorphism. It forms in veins or layers within these host rocks.
Key Facts
- Hardness: 7 (Mohs scale)
- Specific Gravity: 2.64 - 2.71
- Crystal System: Trigonal (as quartz)
- Color: Golden-yellow, golden-brown, reddish-brown, blue (Hawk's Eye)
- Luster: Silky to vitreous, chatoyant
- Transparency: Opaque to translucent
- Fracture: Conchoidal to splintery (due to fibrous structure)
- Cleavage: None
- Composition: Silicon dioxide (SiO2) with fibrous inclusions of altered crocidolite and iron oxides.
Quick Check
- Color: Golden-yellow to reddish-brown (or blue for Hawk's Eye)
- Luster: Silky, chatoyant
- 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
Formation
Tiger's Eye forms when quartz replaces the fibrous amphibole mineral crocidolite (a type of asbestos). This process, known as pseudomorphism, involves the dissolution of crocidolite and simultaneous deposition of chalcedony (microcrystalline quartz), while preserving the original fibrous structure of the crocidolite. Iron oxides (primarily limonite) infiltrate the quartz, imparting the characteristic golden-yellow to reddish-brown colors and contributing to the chatoyancy.
Usage
Primarily used as a gemstone for cabochons, beads, carvings, and ornamental objects due to its attractive chatoyancy. Historically, it was believed to offer protection and bring good luck. It has limited industrial use beyond its aesthetic appeal.
Age Distribution
Precambrian to Cenozoic, depending on the host rock formation.
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 association with banded iron formations (BIFs) or similar iron-rich metamorphic rocks. Prospecting in areas known for BIFs increases the chances of discovery.
Identify Chatoyancy
The most distinctive feature is its chatoyancy. Look for a silky luster and a moving band of light when rotating the specimen under a light source. This is crucial for distinguishing it from other minerals.
Observe Color and Texture
The characteristic golden-yellow to reddish-brown colors and fibrous texture are key indicators. Hawk's Eye (blue Tiger's Eye) will have a bluish hue.
Check Hardness
Tiger's Eye is quartz, so it will scratch glass (Mohs hardness 7). This helps differentiate it from softer fibrous minerals.
Safety Precautions (Crocidolite)
Tiger's Eye is a pseudomorph after crocidolite, a form of asbestos. While the crocidolite has been replaced by quartz, very fine, unreplaced crocidolite fibers can occasionally be present, especially in raw, unpolished specimens. **Always handle raw Tiger's Eye with caution.** Avoid grinding, cutting, or polishing without proper respiratory protection (e.g., P100 respirator) and wet methods to prevent inhalation of potentially hazardous fibers. Polished Tiger's Eye is generally considered safe as the fibers are encapsulated within the quartz matrix.
Similar Rocks
Hawk's Eye
Quartz (fibrous crocidolite pseudomorph)
Also known as: Blue Tiger's Eye
Cat's Eye Chrysoberyl
Chrysoberyl (with fibrous inclusions)
Also known as: Cymophane
Serpentine Cat's Eye
Serpentine (with fibrous inclusions)
Also known as: Serpentine with chatoyancy
Scientific Classification
- Mineral Class
- Silicates (Tectosilicates)
- Group
- Quartz Group
- Crystal System
- Trigonal
- Chemical Formula
- SiO2 (with inclusions)
- Composition
- Silicon dioxide, with fibrous inclusions of altered crocidolite (Na2(Fe2+,Mg)3Fe3+2Si8O22(OH)2) and iron oxides (e.g., limonite).
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