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Tiger's Eye is a captivating metamorphic rock characterized by its silky luster and striking 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. The color ranges from golden-yellow to reddish-brown, often with iridescent bands that shift as the stone is moved. This optical phenomenon, known as chatoyancy, is caused by the reflection of light off the minute, parallel fibers of quartz and iron oxides. It is a relatively hard and durable material, making it suitable for various lapidary applications.
How to Identify
- Color
- Golden-yellow to reddish-brown, often with iridescent bands. Hawk's Eye (blue Tiger's Eye) is blue to bluish-green.
- Luster
- Silky to vitreous, exhibiting strong chatoyancy.
- Texture
- Fibrous appearance, even when polished smooth. Can feel cool and smooth to the touch.
- Crystal Form
- Massive, fibrous aggregates, typically found in veins. Does not form distinct macroscopic crystals.
- Cleavage
- None, as it is a microcrystalline quartz. The original crocidolite had perfect cleavage, but this is not retained in the quartz pseudomorph.
- Geological Environment
- Formed in metamorphic terrains where crocidolite-rich rocks (often associated with banded iron formations) have undergone silicification. Typically found in veins or seams 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, vitreous
- Transparency: Opaque to translucent
- Fracture: Conchoidal to splintery
- Cleavage: None
- Composition: Silicon dioxide (SiO2) with inclusions of iron oxides (limonite, goethite) and relict crocidolite fibers.
Quick Check
- Color: Golden-yellow to reddish-brown (or blue for Hawk's Eye)
- Luster: Silky to vitreous, chatoyant
- Streak: White
Physical Characteristics
- Crystal Habit: Massive, fibrous aggregates, pseudomorph after crocidolite.
- Cleavage Type: None (as quartz). Original crocidolite had perfect cleavage.
- Fracture Type: Conchoidal to splintery.
- Tenacity: Brittle.
- Luster Type: Silky to vitreous, chatoyant.
Formation
Tiger's Eye forms as a pseudomorph of quartz after crocidolite (a fibrous amphibole mineral, specifically riebeckite). The process involves the infiltration of silica-rich solutions into crocidolite veins. The silica replaces the crocidolite fibers while preserving their original fibrous structure. Oxidation of the iron within the crocidolite (or the replacing silica) gives Tiger's Eye its characteristic golden-yellow to reddish-brown colors. The chatoyancy (cat's-eye effect) is due to the parallel alignment of these preserved fibrous structures.
Usage
Primarily used as a gemstone for cabochons, beads, carvings, and ornamental objects. Its unique chatoyancy makes it popular in jewelry. Historically, it was believed to ward off evil and bring good luck.
Age Distribution
Typically found in Precambrian to Paleozoic age metamorphic terrains.
Where to Find
South Africa
The primary and most significant source of high-quality Tiger's Eye, particularly in the Griqualand West region, Northern Cape Province. Deposits are found in Precambrian banded iron formations.
Australia
Significant deposits are found in Western Australia, particularly in the Hamersley Range, often associated with banded iron formations.
India
Smaller deposits are known, producing material of varying quality.
Myanmar (Burma)
Some occurrences have been reported.
United States
Limited occurrences in states like Arizona and California.
Canada
Minor deposits have been found.
Finding Tips
Look for Metamorphic Terrains
Tiger's Eye is found in regions with ancient metamorphic rocks, particularly those associated with banded iron formations.
Identify Fibrous Structures
Even in rough form, look for a distinct fibrous texture. The chatoyancy might be visible even before polishing.
Observe Color and Luster
The characteristic golden-yellow to reddish-brown color and silky luster are key indicators. Blue varieties (Hawk's Eye) also exist.
Check for Chatoyancy
Rotate the specimen under a light source to observe the moving band of light, which is the hallmark of Tiger's Eye.
Hardness Test
Tiger's Eye is quartz, so it should scratch glass (Mohs hardness 7). This helps differentiate it from softer fibrous minerals.
Similar Rocks
Hawk's Eye
Quartz (pseudomorph after Crocidolite)
Also known as: Blue Tiger's Eye
Pietersite
Chalcedony with embedded Crocidolite fibers
Also known as: Tempest Stone
Cat's Eye Chrysoberyl
Chrysoberyl
Also known as: Cymophane
Scientific Classification
- Mineral Class
- Silicates (Tectosilicates)
- Group
- Quartz Group
- Crystal System
- Trigonal
- Chemical Formula
- SiO2 (with fibrous inclusions of FeO(OH) and relict crocidolite)
- Composition
- Silicon dioxide, with the characteristic color and chatoyancy derived from finely fibrous inclusions of iron hydroxides (primarily goethite and limonite) that have replaced the original crocidolite (a sodium-iron amphibole).
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