Chatoyancy (Cat's Eye Effect)
Chatoyancy in gemstones including chrysoberyl cat's eye, the milk and honey effect, and other chatoyant gems.
phenomena/chatoyancy cats-eye species/chrysoberyl cymophane
Introduction
Chatoyancy is a single band of light that moves across a cabochon surface,
resembling a cat's eye. It requires parallel needle or tube inclusions
oriented in one direction, creating reflection along a single axis.
Chrysoberyl cat's eye produces the finest chatoyancy of any gem and is
the only gem that may be called simply "cat's eye" without qualification.
Mechanism
How chatoyancy forms:
Requirements
- Parallel fibrous or tubular inclusions
- Must be cut en cabochon perpendicular to inclusion direction
- Light reflects from the sides of inclusions
- Creates a single bright band (chatoyant band)
Comparison with Asterism
- Chatoyancy: ONE set of parallel inclusions → single band
- Asterism: Multiple sets at angles → star pattern
- Same reflection principle; different inclusion arrangement
Chrysoberyl Cat's Eye
Chrysoberyl cat's eye (cymophane) is the finest and most valuable chatoyant gem.
Characteristics
- Strong, sharp chatoyant band
- Honey yellow to green body colour
- Fine, dense silk creates superior effect
- Only chrysoberyl may be called simply "cat's eye"
- All others require qualification (e.g., "quartz cat's eye")
The Silk
- Extremely fine parallel needle inclusions
- Creates exceptionally sharp eye
- Denser silk = sharper eye
- May be hollow tubes or solid needles
The Milk and Honey Effect
Quality Factors
| Factor | Premium Quality |
|---|---|
| Eye sharpness | Crisp, knife-edge line |
| Eye completeness | Extends girdle to girdle |
| Eye centering | Centred on dome apex |
| Movement | Smooth, fluid motion |
| Milk and honey | Strong contrast when side-lit |
| Body colour | Honey yellow to green |
| Transparency | Semi-translucent (not opaque) |
Other Cat's Eye Gems
| Gem | Typical Colour | Inclusion Type | Notes |
|---|---|---|---|
| Chrysoberyl | Honey yellow, green | Silk needles | Finest; simply 'cat's eye' |
| Quartz (tiger's eye) | Golden brown | Replaced crocidolite | Chatoyant fibrous quartz |
| Tourmaline | Various | Growth tubes | Good eyes possible |
| Apatite | Yellow-green | Needle inclusions | Collector interest |
| Sillimanite | Grey-green | Fibrous habit | Less common |
| Scapolite | Yellow, pink | Needle inclusions | Attractive variety |
| Actinolite | Green | Fibrous nature | Often marketed as 'cat's eye jade' |
Tiger's Eye
Tiger's eye quartz is the most common chatoyant gem:
Formation
- Originally fibrous crocidolite (blue asbestos)
- Replaced by quartz while preserving fibrous structure
- Retains golden-brown colour from iron staining
Varieties
| Name | Colour | Notes |
|---|---|---|
| Tiger's eye | Golden brown | Iron-stained; most common |
| Hawk's eye | Blue-grey | Less altered; retains blue |
| Tiger iron | Banded | Alternating with hematite/jasper |
Nomenclature Rules
Cutting Considerations
Proper cutting maximises chatoyancy:
- Orientation: Perpendicular to needle direction
- Dome shape: Adequate height for light interaction
- Centering: Eye should appear centred
- Symmetry: Even cabochon shape
- Base: Properly proportioned