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Blue Tiger's Eye is a captivating variety of quartz characterized by its striking blue-gray to blue-green color and prominent chatoyancy. This optical phenomenon, resembling the slit pupil of a cat's or hawk's eye, is caused by the reflection of light off parallel fibrous inclusions of crocidolite within the quartz. Unlike the more common golden-brown Tiger's Eye, Blue Tiger's Eye retains the original blue-gray color of the crocidolite fibers because the iron within them has not undergone significant oxidation. It is a pseudomorph, meaning one mineral (quartz) has replaced another (crocidolite) while retaining the original mineral's external form and internal fibrous structure.
How to Identify
- Color
- Blue-gray to blue-green, often with darker bands. The color is uniform within the fibrous bands.
- Luster
- Silky to vitreous, especially on polished surfaces, exhibiting strong chatoyancy.
- Texture
- Fibrous internal structure, often visible as parallel lines or bands. Smooth to the touch when polished.
- Crystal Form
- Typically massive, occurring as veins or lenses. Individual quartz crystals are microscopic (cryptocrystalline).
- Cleavage
- None, as it is a cryptocrystalline quartz.
- Geological Environment
- Found in metamorphosed iron formations, particularly those rich in crocidolite. It forms in areas where silica-rich fluids have permeated and replaced the original crocidolite asbestos.
Key Facts
- Hardness: 6.5-7 on the Mohs scale
- Specific Gravity: 2.64-2.71
- Crystal System: Trigonal (cryptocrystalline)
- Color: Blue-gray, blue-green, sometimes with golden-brown streaks
- Luster: Silky to vitreous, chatoyant
- Transparency: Opaque to translucent
- Fracture: Conchoidal to splintery
- Cleavage: None
- Composition: Silicon dioxide (SiO2) with embedded crocidolite fibers
Quick Check
- Color: Blue-gray to blue-green
- Luster: Silky to vitreous, chatoyant
- Streak: White
Physical Characteristics
- Crystal Habit: Massive, fibrous aggregates, pseudomorphs after crocidolite
- Cleavage Type: None
- Fracture Type: Conchoidal to splintery, due to the cryptocrystalline nature and fibrous inclusions
- Tenacity: Brittle
- Luster Type: Silky (due to fibrous inclusions) to vitreous (of the quartz matrix)
Formation
Blue Tiger's Eye, or Hawk's Eye, is a pseudomorph of quartz after crocidolite, a fibrous amphibole mineral (Na2(Fe2+,Mg)3Fe3+2Si8O22(OH)2). The formation process involves the infiltration of silica-rich hydrothermal solutions into crocidolite deposits. The silica replaces the crocidolite fibers while preserving their original fibrous structure. The blue-gray to blue-green color and chatoyancy (the 'eye' effect) are due to the presence of unoxidized or partially oxidized crocidolite fibers embedded within the quartz. As oxidation progresses, the iron in the crocidolite oxidizes, leading to the golden-brown color of classic Tiger's Eye. Therefore, Blue Tiger's Eye represents an earlier stage of this pseudomorphic replacement and oxidation process.
Usage
Primarily used as an ornamental stone, for cabochons, beads, carvings, and other lapidary applications. It is also popular in jewelry due to its unique chatoyancy and attractive color. Historically, it has been used for protective amulets.
Age Distribution
Typically found in Precambrian to Cenozoic metamorphic terrains, depending on the age of the host rock and the crocidolite formation.
Where to Find
South Africa
The primary and most significant source, particularly in the Northern Cape Province, where vast deposits of crocidolite-bearing iron formations occur.
Western Australia
Found in the Hamersley Range, often associated with banded iron formations.
Namibia
Occurrences are noted, often linked to similar geological settings as South Africa.
United States
Limited occurrences, primarily in Arizona and California, though less commercially significant than African sources.
Finding Tips
Look for Metamorphic Terrains
Focus on regions known for metamorphosed iron formations or areas with historical crocidolite (blue asbestos) mining, as these are the host rocks.
Identify Fibrous Structure
Even in rough form, look for a distinct parallel fibrous texture. This is key to identifying the pseudomorph.
Observe Chatoyancy
Rotate the specimen under a light source. The characteristic 'eye' effect (chatoyancy) should be evident, moving across the surface.
Check Color
The blue-gray to blue-green color is distinctive. Be aware that some material may show partial oxidation, leading to patches of golden-brown.
Safety Precautions
Blue Tiger's Eye is a pseudomorph of quartz after crocidolite. While the crocidolite fibers are encapsulated in quartz and generally considered safe in polished or stable forms, handling raw, unpolished material, especially if it is friable or dusty, carries a risk of exposure to asbestos fibers. Crocidolite is a known carcinogen. Always wear appropriate respiratory protection (P100 respirator) and eye protection when working with rough or unpolished specimens, and avoid creating dust. Do not saw, grind, or sand raw material without proper ventilation and personal protective equipment. Wash hands thoroughly after handling. Polished specimens are generally safe as the fibers are encased.
Similar Rocks
Tiger's Eye
Quartz (variety of Tiger's Eye)
Also known as: Golden Tiger's Eye
Pietersite
Chalcedony with embedded crocidolite fibers
Also known as: Tempest Stone
Serpentine
Mg3Si2O5(OH)4
Also known as: New Jade
Scientific Classification
- Mineral Class
- Silicates (Tectosilicates)
- Group
- Quartz Group
- Crystal System
- Trigonal
- Chemical Formula
- SiO2 (with inclusions of Na2(Fe2+,Mg)3Fe3+2Si8O22(OH)2)
- Composition
- Silicon dioxide, with fibrous inclusions of unoxidized or partially oxidized crocidolite (a sodium-iron amphibole)
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