Optical Phenomena Overview
Introduction to optical phenomena in gemstones including light interaction principles and summary of major phenomena.
Introduction
Phenomenal gems display special optical effects caused by their internal structure,
inclusions, or the interference, diffraction, or reflection of light within the
stone. These phenomena are among the most commercially significant properties in
gemmology: a well-centred star in a fine ruby, a sharp cat's eye in chrysoberyl,
or a complete colour change in alexandrite can multiply a stone's value over a
non-phenomenal specimen of otherwise equal quality. [1]
The major phenomena divide by physical cause. Reflection effects (asterism,
chatoyancy, and aventurescence) arise from light bouncing off oriented or platy
inclusions. Interference effects (adularescence, labradorescence, schiller, and
iridescence) arise from wave interactions at thin layers within the stone.
Diffraction effects (play of colour in opal) arise from periodic structures at
the scale of visible wavelengths. Absorption effects (colour change) arise from
selective transmission through chromophore bands.
Understanding the physical cause of each phenomenon enables correct identification,
quality assessment, and separation of natural from synthetic or treated material.
Types of Optical Phenomena
Gem phenomena can be grouped by their physical causes:
Reflection Effects
Caused by light reflecting from internal features:
- Asterism: Star patterns from oriented needle inclusions
- Chatoyancy: Cat's eye band from parallel inclusions
- Aventurescence: Metallic sparkle from platy inclusions
Interference Effects
Caused by light interference between thin layers:
- Adularescence: Soft glow in moonstone (feldspar layers)
- Labradorescence: Colour flashes from twinning (labradorite)
- Iridescence: Rainbow colours from thin films
Diffraction Effects
Caused by light diffraction from regular structures:
- Play of colour: Spectral colours in opal (silica spheres)
Absorption Effects
Caused by selective absorption of light:
- Colour change: Different colours under different lights
Phenomena Summary
| Phenomenon | Cause | Key Gems | Effect Appearance |
|---|---|---|---|
| Asterism | Oriented needle inclusions | Star ruby, star sapphire | 4-, 6-, or 12-ray star |
| Chatoyancy | Parallel needle/tube inclusions | Chrysoberyl cat's eye | Single moving band |
| Adularescence | Feldspar layer interference | Moonstone | Floating blue-white glow |
| Labradorescence | Lamellar twinning interference | Labradorite, spectrolite | Multicoloured flashes |
| Play of colour | Silica sphere diffraction | Precious opal | Shifting spectral colours |
| Colour change | Selective absorption spectrum | Alexandrite | Different colour in daylight/incandescent |
| Aventurescence | Reflective platy inclusions | Sunstone, aventurine | Metallic sparkle |
| Iridescence | Thin film interference | Fire agate, ammolite | Rainbow colours |
Value Impact
Phenomena can significantly affect gem value:
Premium Factors
- Strength: More pronounced effects command higher prices
- Quality: Sharp, centred, complete effects preferred
- Rarity: Uncommon phenomena in unusual gems valued
- Body colour: Attractive underlying colour adds value
Quality Assessment
For phenomenal gems, assess:
- Is the effect sharp and well-defined?
- Is it centred and complete?
- Does it move smoothly?
- Is the body colour attractive?
- Is the stone well cut to display the effect?
Cutting for Phenomena
References
- ↑ 1. Read, P. (2014). Gemmology (3rd ed.). Butterworth-Heinemann/Routledge. DOI: 10.4324/9780080507224.