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Cat's Eye Quartz is a variety of quartz that exhibits a phenomenon known as chatoyancy, or the 'cat's eye effect'. This optical effect is characterized by a single, bright, silky band of light that appears to move across the surface of the stone when it is rotated or when the light source moves. This effect is caused by the reflection of light from numerous parallel, needle-like inclusions or hollow tubes within the quartz. The color of Cat's Eye Quartz can vary widely, but it is most commonly found in shades of gray, green, yellow, brown, or white. The quality of the cat's eye effect is determined by the sharpness and mobility of the light band.
How to Identify
- Color
- Variable, commonly gray, green, yellow, brown, white. The color is often influenced by the type of fibrous inclusions.
- Luster
- Vitreous (glassy) to greasy, with a silky sheen on the chatoyant band.
- Texture
- Typically massive or granular, often polished into a smooth cabochon to best display the chatoyancy.
- Crystal Form
- Usually found as massive aggregates or in veins. Individual crystals are typically prismatic, but the cat's eye effect is best observed in polished, non-faceted forms.
- Cleavage
- None, due to its cryptocrystalline or massive nature.
- Geological Environment
- Hydrothermal veins, pegmatites, and metamorphic rocks where conditions allow for the growth of quartz with oriented fibrous inclusions (e.g., rutile, amphibole).
Key Facts
- Hardness: 7 on the Mohs scale
- Specific Gravity: 2.65 g/cm³
- Crystal System: Trigonal
- Color: Variable, commonly gray, green, yellow, brown, white
- Luster: Vitreous to greasy, silky on chatoyant band
- Transparency: Translucent to opaque
- Fracture: Conchoidal
- Cleavage: None
- Composition: Silicon dioxide (SiO2) with fibrous inclusions
Quick Check
- Color: Variable (gray, green, yellow, brown, white)
- Luster: Vitreous to greasy, silky on chatoyant band
- Streak: White
Physical Characteristics
- Crystal Habit: Massive, granular, or prismatic crystals (though chatoyancy is best in massive forms cut as cabochons)
- Cleavage Type: None
- Fracture Type: Conchoidal
- Tenacity: Brittle
- Luster Type: Vitreous to greasy, silky
Formation
Cat's Eye Quartz forms in pegmatites, hydrothermal veins, and metamorphic rocks. The chatoyancy (cat's eye effect) is caused by parallel fibrous inclusions, typically of rutile (TiO2) or amphibole minerals (e.g., crocidolite, actinolite), or sometimes hollow channels, oriented perpendicular to the c-axis of the quartz crystal. When polished into a cabochon, these inclusions reflect light in a single band, creating the characteristic optical phenomenon.
Usage
Primarily used as a gemstone, particularly for cabochons, beads, and carvings due to its unique chatoyancy. It is also collected by mineral enthusiasts.
Age Distribution
Formed throughout geological history, wherever quartz-bearing rocks are found and conditions for fibrous inclusions exist.
Where to Find
Brazil
Significant source of various quartz varieties, including Cat's Eye Quartz.
India
Known for producing good quality Cat's Eye Quartz.
Sri Lanka
Historically a major source of high-quality gemstones, including Cat's Eye Quartz and Cat's Eye Chrysoberyl.
United States (e.g., North Carolina, California)
Localities where quartz with chatoyancy has been found.
Madagascar
Produces various quartz types, including some with chatoyancy.
Finding Tips
Look for Chatoyancy
The defining characteristic is the moving band of light. Rotate the specimen under a single light source to observe this effect. This is best seen on polished, domed surfaces (cabochons).
Examine Inclusions
Under magnification, fine, parallel, needle-like inclusions or channels should be visible, responsible for the chatoyancy. These are typically rutile or amphibole fibers.
Hardness Test
Quartz has a Mohs hardness of 7. It should scratch glass and not be easily scratched by a steel knife. This helps distinguish it from softer minerals.
Specific Gravity
Quartz has a specific gravity of 2.65. This can be used to differentiate it from other chatoyant minerals with different densities.
Distinguish from Cat's Eye Chrysoberyl
Cat's Eye Chrysoberyl (Mohs hardness 8.5) is significantly harder and typically has a sharper, more defined 'eye' and a higher specific gravity (3.71-3.75). Cat's Eye Quartz is also generally less expensive.
Similar Rocks
Cat's Eye Chrysoberyl
Chrysoberyl (variety: Cat's Eye)
Also known as: Cymophane
Tiger's Eye
Quartz (variety: Tiger's Eye)
Also known as: Tiger Eye
Hawk's Eye
Quartz (variety: Hawk's Eye)
Also known as: Blue Tiger's Eye
Scientific Classification
- Mineral Class
- Silicates
- Group
- Tectosilicates (Quartz Group)
- Crystal System
- Trigonal
- Chemical Formula
- SiO2
- Composition
- Silicon dioxide with fibrous inclusions (e.g., rutile, amphibole)
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