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Cat's Eye Chrysoberyl, or Cymophane, is a rare and highly valued gemstone variety of the mineral chrysoberyl. Its most distinctive feature is a sharp, luminous band of light that appears to glide across the surface of the stone when it is moved, resembling the slit pupil of a cat's eye. This optical effect, known as chatoyancy, is caused by microscopic, parallel fibrous inclusions or hollow tubes within the crystal. The color typically ranges from honey-yellow to yellowish-green, green, and brownish-yellow. It is a beryllium aluminum oxide mineral.
How to Identify
- Color
- Typically honey-yellow, yellowish-green, green, or brownish-yellow. Can also be gray or brown.
- Luster
- Vitreous to greasy.
- Texture
- Smooth when polished; rough on fracture surfaces.
- Crystal Form
- Orthorhombic system, often forming tabular or short prismatic crystals, frequently twinned, especially in pseudohexagonal forms. When cut en cabochon, the chatoyant effect is visible.
- Cleavage
- Distinct on {110}, indistinct on {010} and {001}.
- Geological Environment
- Found in granitic pegmatites, mica schists, and sometimes in contact metamorphic deposits. Due to its high specific gravity and hardness, it is also concentrated in alluvial and eluvial placer deposits.
Key Facts
- Hardness: 8.5 (Mohs scale)
- Specific Gravity: 3.71 - 3.75
- Crystal System: Orthorhombic
- Color: Yellow, yellowish-green, green, brownish-yellow, gray, brown
- Luster: Vitreous to greasy
- Transparency: Transparent to translucent
- Fracture: Conchoidal to uneven
- Cleavage: Distinct on {110}, indistinct on {010} and {001}
- Composition: Beryllium Aluminum Oxide (BeAl2O4)
Quick Check
- Color: Honey-yellow to yellowish-green
- Luster: Vitreous to greasy
- Streak: White
Physical Characteristics
- Crystal Habit: Tabular, short prismatic, often twinned (pseudohexagonal), massive, granular. Chatoyancy is best observed in cabochon cuts.
- Cleavage Type: Distinct in one direction ({110}), indistinct in two others ({010}, {001}).
- Fracture Type: Conchoidal to uneven, brittle.
- Tenacity: Brittle
- Luster Type: Vitreous to greasy
Formation
Chrysoberyl forms in pegmatites, granites, and mica schists. It can also be found in alluvial and eluvial deposits due to its hardness and resistance to weathering. The cat's eye effect (chatoyancy) in cymophane is caused by the reflection of light from parallel needle-like inclusions (typically rutile or hollow tubes) within the crystal structure. These inclusions are oriented perpendicular to the c-axis of the chrysoberyl crystal.
Usage
Primarily used as a gemstone, highly prized for its unique chatoyancy. It is often cut en cabochon to best display this optical phenomenon. Due to its hardness (8.5 on Mohs scale), it is suitable for all types of jewelry.
Age Distribution
Precambrian to Cenozoic, depending on the host rock formation.
Where to Find
Sri Lanka (Ceylon)
Historically the most important source, particularly for fine quality, honey-colored cat's eye chrysoberyl. Found in gem gravels.
Brazil
Minas Gerais state is a significant producer, yielding good quality material.
India
Known for producing cat's eye chrysoberyl, particularly in the southern states.
Madagascar
Emerging source for chrysoberyl, including cat's eye varieties.
Tanzania
Produces chrysoberyl, sometimes with chatoyancy.
Russia (Ural Mountains)
Historically known for chrysoberyl, including some cat's eye material.
Finding Tips
Look in Pegmatites
Search for chrysoberyl in granitic pegmatite dikes, especially those associated with mica schists.
Check Alluvial Deposits
Due to its hardness and density, chrysoberyl often accumulates in placer deposits (riverbeds, ancient stream channels). Look for heavy mineral concentrations.
Examine Host Rocks
Identify areas with known chrysoberyl occurrences. Look for associated minerals like quartz, feldspar, mica, and garnet.
Use a Loupe
When examining rough material, use a loupe to look for the characteristic fibrous inclusions that cause chatoyancy. The effect is best seen under a single light source.
Test Hardness
Chrysoberyl is very hard (8.5 Mohs). This can help distinguish it from softer minerals with similar appearances.
Similar Rocks
Tiger's Eye
Quartz (variety macrocrystalline)
Also known as: Crocodilite
Hawk's Eye
Quartz (variety macrocrystalline)
Also known as: Blue Tiger's Eye
Cat's Eye Tourmaline
Tourmaline (variety)
Also known as: Chatoyant Tourmaline
Cat's Eye Apatite
Apatite (variety)
Also known as: Chatoyant Apatite
Scientific Classification
- Mineral Class
- Oxides
- Group
- Chrysoberyl Group
- Crystal System
- Orthorhombic
- Chemical Formula
- BeAl2O4
- Composition
- Beryllium Aluminum Oxide
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