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Tiger's Eye is a captivating gemstone known for its silky luster and characteristic chatoyancy, resembling the eye of a tiger. It is a pseudomorph of quartz after crocidolite, meaning quartz has replaced the original crocidolite fibers while retaining their parallel, fibrous structure. This structure, combined with the presence of iron oxides, creates the distinctive golden-yellow to reddish-brown colors and the shimmering band of light that moves across the surface as the stone is rotated. It is a variety of chalcedony, which itself is a cryptocrystalline form of quartz.
How to Identify
- Color
- Golden-yellow, golden-brown, reddish-brown, sometimes with darker bands. Hawk's Eye (blue Tiger's Eye) is blue to bluish-green.
- Luster
- Silky to vitreous, exhibiting strong chatoyancy.
- Texture
- Fine-grained, fibrous appearance internally, often polished smooth externally.
- Crystal Form
- Massive, fibrous aggregates, typically found as veins or layers within host rock. Does not form macroscopic crystals.
- Cleavage
- None, due to its cryptocrystalline nature.
- Geological Environment
- Typically found in highly metamorphosed iron formations, often associated with asbestos deposits. It forms in veins or layers within these rocks where crocidolite has been altered by silica-rich hydrothermal fluids.
Key Facts
- Hardness: 6.5-7 on the 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
- Cleavage: None
- Composition: Silicon dioxide (SiO2) with fibrous inclusions of iron-stained amphibole (originally crocidolite)
Quick Check
- Color: Golden-yellow to reddish-brown with iridescent bands.
- Luster: Silky, chatoyant.
- Streak: White
Physical Characteristics
- Crystal Habit: Massive, fibrous aggregates, pseudomorphous after crocidolite.
- Cleavage Type: None
- Fracture Type: Conchoidal to splintery, due to its cryptocrystalline nature and fibrous structure.
- Tenacity: Brittle
- Luster Type: Silky (due to fibrous inclusions) to vitreous (of the quartz matrix)
Formation
Tiger's Eye forms as a pseudomorph of quartz after crocidolite (a fibrous blue asbestos mineral). The process involves the infiltration of silica-rich solutions into crocidolite, where the silica replaces the crocidolite fibers while preserving their original fibrous structure. Oxidation of the iron within the crocidolite (or the replacing silica) leads to the characteristic golden-yellow to reddish-brown colors. The chatoyancy (cat's eye effect) is due to the parallel alignment of these preserved fibrous inclusions.
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 metamorphic terrains, with specific deposits varying in age.
Where to Find
South Africa
The primary and most significant source, particularly in the Northern Cape Province, where vast deposits are found in the Griqualand West region. These deposits are associated with the Asbestos Hills and the Kuruman Iron Formation.
Australia
Significant deposits are found in Western Australia, particularly in the Hamersley Range, often associated with banded iron formations.
India
Smaller deposits are known in various regions.
Myanmar (Burma)
Some occurrences have been reported.
United States
Limited occurrences, primarily in Arizona and California.
Finding Tips
Look for Metamorphic Terrains
Tiger's Eye is found in metamorphic rocks, particularly those associated with ancient iron formations and former asbestos deposits. Focus your search in areas known for such geology.
Identify Fibrous Structures
Even in its raw form, look for a distinct fibrous texture. The chatoyancy might not be as pronounced until polished, but the parallel fibers are key.
Check for Associated Minerals
Tiger's Eye often occurs with other minerals found in iron formations, such as hematite, magnetite, and various amphiboles. The presence of these can indicate a potential Tiger's Eye deposit.
Safety Precautions (Crocidolite)
Tiger's Eye is a pseudomorph after crocidolite, a form of asbestos. While the silica replacement generally renders it safe, handling raw, unpolished material, especially if friable or dusty, should be done with caution. Avoid inhaling dust. Always wear appropriate personal protective equipment (PPE), including a respirator, when collecting or processing raw material from known crocidolite-bearing areas. Polished Tiger's Eye is considered safe.
Similar Rocks
Hawk's Eye
Hawk's Eye (variety of Chalcedony, a cryptocrystalline form of Quartz)
Also known as: Blue Tiger's Eye
Pietersite
Pietersite (brecciated chalcedony with embedded amphibole fibers)
Also known as: Tempest Stone
Cat's Eye Chrysoberyl
Chrysoberyl (BeAl2O4)
Also known as: Cymophane
Scientific Classification
- Mineral Class
- Silicates (Tectosilicates)
- Group
- Quartz Group (Chalcedony variety)
- Crystal System
- Trigonal (microcrystalline)
- Chemical Formula
- SiO2 (with inclusions)
- Composition
- Silicon dioxide with oriented fibrous inclusions of iron-stained amphibole (primarily goethite and hematite after crocidolite).
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