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Hawk's Eye is a variety of quartz that exhibits a striking chatoyancy, or 'cat's-eye' effect, appearing as a silky band of light that moves across the surface when the stone is rotated. Its color ranges from dark blue-gray to blue-green, often with subtle iridescent flashes. It is essentially an unoxidized or less oxidized precursor to the more commonly known golden Tiger's Eye. The fibrous structure responsible for its optical properties is derived from the mineral crocidolite, which has been replaced by quartz.
How to Identify
- Color
- Dark blue-gray to blue-green, sometimes with hints of brown or black. The blue hue is distinct from the golden-brown of classic Tiger's Eye.
- Luster
- Silky to vitreous, especially on polished surfaces, due to the fibrous inclusions.
- Texture
- Fine-grained, fibrous structure visible under magnification, leading to a smooth feel when polished.
- Crystal Form
- Typically massive, cryptocrystalline aggregates, rarely showing macroscopic crystal forms of quartz. The fibrous structure is a pseudomorph after crocidolite.
- Cleavage
- None, as it is a cryptocrystalline quartz. It exhibits conchoidal fracture.
- Geological Environment
- Found in metamorphosed iron formations, particularly those rich in crocidolite. It forms in veins or layers within these host rocks.
Key Facts
- Hardness: 7 on the Mohs scale (due to quartz).
- Specific Gravity: 2.64 - 2.71 g/cm³.
- Crystal System: Trigonal (for quartz), but the macroscopic form is massive fibrous.
- Color: Blue-gray, blue-green, sometimes with dark streaks.
- Luster: Silky to vitreous.
- Transparency: Opaque.
- Fracture: Conchoidal to splintery (due to fibrous structure).
- Cleavage: None.
- Composition: Silicon dioxide (SiO₂) with embedded, unoxidized crocidolite fibers.
Quick Check
- Color: Blue-gray to blue-green with chatoyancy.
- Luster: Silky to vitreous.
- Streak: White (due to quartz).
Physical Characteristics
- Crystal Habit: Massive, fibrous aggregates, pseudomorphous after crocidolite.
- Cleavage Type: None.
- Fracture Type: Conchoidal to splintery.
- Tenacity: Brittle.
- Luster Type: Silky to vitreous.
Formation
Hawk's Eye forms through a process of pseudomorphism, where fibrous crocidolite (a blue asbestos mineral, specifically riebeckite) is replaced by quartz while retaining the original fibrous structure. This replacement occurs under low-temperature, low-pressure metamorphic conditions, often in iron-rich host rocks. The blue-gray to blue-green color is due to the presence of unoxidized crocidolite fibers, which are typically more abundant and less oxidized than in classic golden Tiger's Eye. The chatoyancy (cat's-eye effect) is a result of the parallel alignment of these relict crocidolite fibers within the quartz matrix, which reflect light in a silky band.
Usage
Primarily used as a gemstone for cabochons, beads, carvings, and ornamental objects due to its attractive chatoyancy and color. It is also collected by mineral enthusiasts.
Age Distribution
Precambrian to Cenozoic, depending on the host rock formation.
Where to Find
South Africa
The primary source, particularly in the Griqualand West region, where extensive deposits of crocidolite-rich iron formations occur.
Australia
Western Australia, specifically the Hamersley Range, is another significant source.
Myanmar (Burma)
Known for some occurrences, though less prominent than South Africa or Australia.
India
Limited occurrences have been reported.
United States
Minor deposits in certain iron-rich metamorphic terrains.
Finding Tips
Geological Context
Look for Hawk's Eye in areas known for metamorphosed banded iron formations (BIFs) or other iron-rich sedimentary rocks that have undergone low-grade metamorphism.
Associated Minerals
It is often found alongside other forms of quartz, jasper, and iron oxides. The presence of relict crocidolite fibers is key.
Chatoyancy
The most distinctive feature is its chatoyancy. Rotate potential specimens under a light source to observe the moving band of light. This effect is best seen on polished or tumbled surfaces.
Color Variation
While predominantly blue-gray to blue-green, some specimens may show areas transitioning to golden-brown Tiger's Eye, indicating varying degrees of crocidolite oxidation.
Safety Precaution
As Hawk's Eye is a pseudomorph after crocidolite (a form of asbestos), raw, unpolished specimens, especially those with visible loose fibers, should be handled with caution. Avoid inhaling dust if cutting or grinding. Polished specimens are generally considered safe as the asbestos fibers are encapsulated within the quartz matrix.
Similar Rocks
Tiger's Eye
Quartz (pseudomorph after Crocidolite)
Also known as: Golden Tiger's Eye
Pietersite
Chalcedony with embedded amphibole 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 (for quartz)
- Chemical Formula
- SiO₂ (with inclusions of Na₂Fe²⁺₃Fe³⁺₂Si₈O₂₂(OH)₂ for crocidolite)
- Composition
- Silicon dioxide, with fibrous inclusions of unoxidized crocidolite (a sodium-iron amphibole).
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