Chatoyancy (Cat's Eye Effect)
Chatoyancy in gemstones including chrysoberyl cat's eye, the milk and honey effect, and other chatoyant gems.
Introduction
Chatoyancy (the cat's eye effect) is a single band of light that appears to
glide across a cabochon surface when the stone is rotated under a point light
source. It requires two conditions: a high concentration of parallel fibrous,
tubular, or needle-like inclusions oriented in one direction, and a cabochon
cut perpendicular to that inclusion direction. Light reflects from the sides
of the inclusions and concentrates along one axis, creating the band.
[1]
Chrysoberyl cat's eye produces the strongest chatoyancy of any gem and is
the only variety that may be called simply "cat's eye" without qualification;
all other chatoyant gems must be named (e.g., "quartz cat's eye",
"tourmaline cat's eye"). The finest specimens show the "milk and honey"
effect: one half of the stone appears milky white, the other honey-gold.
[2][3]
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. [1]
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 [1]
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
References
- ↑ 1. Read, P. (2014). Gemmology (3rd ed.). Butterworth-Heinemann/Routledge. DOI: 10.4324/9780080507224.
- ↑ 2. Nassau, K. (2001). The Physics and Chemistry of Color (2nd ed.). Wiley-Interscience. ISBN: 978-0-471-39106-7.
- ↑ 3. Schumann, W. (2009). Gemstones of the World (4th ed.). Sterling. ISBN: 978-1-4027-6829-3.