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Red Tiger's Eye is a macro-crystalline variety of quartz characterized by its striking reddish-brown to brick-red color and a distinct chatoyancy (cat's eye effect). This optical phenomenon is caused by the reflection of light off parallel fibrous inclusions, which are pseudomorphs of quartz after crocidolite asbestos. The red coloration distinguishes it from the more common golden-brown Tiger's Eye and the blue Hawk's Eye (Pietersite is a brecciated form of Hawk's Eye and Tiger's Eye).
How to Identify
- Color
- Ranges from reddish-brown to brick-red, often with darker bands. The color is due to oxidized iron inclusions.
- Luster
- Silky to vitreous when polished, exhibiting a strong chatoyancy.
- Texture
- Fine-grained, fibrous appearance internally, but feels smooth when polished. The fibrous structure is visible as parallel lines or bands.
- Crystal Form
- Massive, cryptocrystalline aggregates, typically forming veins or layers. Does not form macroscopic individual crystals.
- Cleavage
- None, due to its cryptocrystalline nature. Exhibits conchoidal fracture.
- Geological Environment
- Found in highly metamorphosed banded iron formations, where crocidolite asbestos has undergone silicification and subsequent oxidation. Often occurs in association with other quartz varieties and iron-rich minerals.
Key Facts
- Hardness: 7 (Mohs scale)
- Specific Gravity: 2.64 - 2.71
- Crystal System: Trigonal
- Color: Reddish-brown to brick-red
- Luster: Silky to vitreous
- Transparency: Opaque to translucent
- Fracture: Conchoidal to splintery
- Cleavage: None
- Composition: Silicon dioxide (SiO2) with fibrous inclusions of iron oxides (hematite) pseudomorphing after crocidolite
Quick Check
- Color: Reddish-brown to brick-red with parallel bands
- Luster: Silky to vitreous, strong chatoyancy
- Streak: White
Physical Characteristics
- Crystal Habit: Massive, fibrous aggregates, pseudomorphous after crocidolite
- Cleavage Type: None
- Fracture Type: Conchoidal to splintery
- Tenacity: Brittle
- Luster Type: Silky (due to fibrous inclusions) to vitreous (of the quartz matrix)
Formation
Red Tiger's Eye forms when fibrous crocidolite (a blue asbestos mineral) is replaced by quartz, while retaining the original fibrous structure. This process is known as pseudomorphism. The red color is primarily due to the oxidation of iron within the original crocidolite fibers, transforming the iron hydroxides (like goethite) into iron oxides (like hematite). This oxidation can occur naturally through weathering processes or can be induced by heat treatment.
Usage
Primarily used as an ornamental stone, for cabochons, beads, carvings, and other lapidary applications. Its chatoyancy makes it highly desirable in jewelry. Historically, it has been used for protective amulets.
Age Distribution
Typically associated with Precambrian banded iron formations, but the silicification and oxidation processes can occur over geological timescales.
Where to Find
South Africa
The primary source of high-quality Red Tiger's Eye, particularly in the Northern Cape Province, where extensive banded iron formations host the original crocidolite deposits.
Australia
Western Australia, particularly the Hamersley Range, is another significant source, often found in association with iron ore deposits.
India
Some occurrences have been reported, though generally less prominent than South African or Australian sources.
Finding Tips
Look for Chatoyancy
The most distinctive feature is the silky, shifting band of light (chatoyancy) that moves across the surface when the stone is rotated. This is crucial for identification.
Observe Color and Banding
Red Tiger's Eye exhibits reddish-brown to brick-red hues, often with parallel bands of varying intensity. The color should be consistent with oxidized iron.
Check for Fibrous Structure
Even in polished specimens, the underlying fibrous structure, which causes the chatoyancy, can often be discerned upon close inspection.
Hardness Test
As a quartz variety, it should scratch glass (Mohs hardness 7). This helps differentiate it from softer imitations.
Distinguish from Heat-Treated Tiger's Eye
Much of the Red Tiger's Eye on the market is heat-treated golden Tiger's Eye. While chemically identical, naturally occurring Red Tiger's Eye often has a more nuanced color distribution. Distinguishing between natural and heat-treated can be challenging without advanced mineralogical analysis, but for collectors, it's an important distinction.
Similar Rocks
Tiger's Eye
Quartz (macro-crystalline variety), pseudomorph after crocidolite
Also known as: Golden Tiger's Eye
Hawk's Eye
Quartz (macro-crystalline variety), pseudomorph after crocidolite
Also known as: Blue Tiger's Eye
Pietersite
Brecciated Hawk's Eye and Tiger's Eye
Also known as: Tempest Stone
Jasper
Cryptocrystalline Quartz
Also known as: Red Jasper
Scientific Classification
- Mineral Class
- Silicates
- Group
- Tectosilicates (Quartz Group)
- Crystal System
- Trigonal
- Chemical Formula
- SiO2 (with Fe2O3 inclusions)
- Composition
- Silicon dioxide with iron oxide inclusions (hematite) that are pseudomorphs after crocidolite (Na2(Fe2+,Mg)3Fe3+2Si8O22(OH)2)
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