Play of Colour
Play of colour in precious opal including cause, pattern types, body tones, and quality assessment.
Introduction
Play of colour is the shifting display of pure spectral colours that
distinguishes precious opal from colourless or single-hue common opal
(potch). It is produced by diffraction: the opal structure consists of
uniform silica spheres (SiO₂) between 150 and 400 nm in diameter, stacked
in a close-packed three-dimensional array. This periodic lattice acts as a
diffraction grating for visible light, selectively reflecting wavelengths
determined by sphere size. [1]
Larger spheres (350 nm) diffract red light, the rarest and most valuable150 nm) produce blue or violet.
colour in opal; smaller spheres (
Uniformity of sphere size and regularity of stacking determine the
brightness of the effect; disorder in either produces potch rather than
precious opal. The pattern name, whether harlequin (angular mosaic patches),
rolling flash, or pinfire, and colour range both influence value substantially.
[2] Black-body opal, in which a dark background
maximises colour contrast, consistently commands the highest prices.
[3]
Cause
The physics of play of colour:
Silica Sphere Structure
Precious vs Common Opal
Precious Opal
- Shows play of colour
- Uniform sphere size
- Regularly stacked spheres
- 3D ordered structure
- Gem quality
Common Opal (Potch)
- No play of colour
- Variable sphere sizes
- Randomly arranged spheres
- Disordered structure
- Not gem quality
Pattern Types
Opal patterns significantly affect value:
| Pattern | Description | Relative Value |
|---|---|---|
| Harlequin | Large angular mosaic patches | Most valuable |
| Flagstone | Large irregular patches | High |
| Floral | Flower-like patterns | High |
| Rolling flash | Large colour areas that shift | Medium-high |
| Broad flash | Single large colour area | Medium |
| Pinfire | Small dense points of colour | Medium |
| Flame | Sweeping streaks of colour | Medium |
| Straw/grass | Thin parallel lines | Lower |
Harlequin Pattern
Colour Value Hierarchy
Not all colours are equally valued:
Colour Rankings
- Red: Rarest (requires largest spheres); most valuable
- Orange: Second rarest
- Green: Middle value
- Blue: Most common play of colour; lower value
An opal showing red (especially with other colours) is more
valuable than one showing only blue.
Colour Range
- Full spectrum: Shows all colours; highly valued
- Partial spectrum: Missing some colours; moderate
- Single colour: Only one colour; lower value (usually blue)
Body Tone Impact
Background colour affects value:
| Body Tone | Effect on Play of Colour | Value Impact |
|---|---|---|
| Black (N1-N4) | Colours appear most vivid | Highest value |
| Dark (N5-N6) | Good colour contrast | High value |
| Light (N7-N9) | Softer appearance | Moderate value |
| Crystal | Transparent; colours from both sides | Variable; can be high |
Quality Assessment
Evaluating play of colour quality:
- Brightness: How vivid are the colours?
- Colour range: How many colours present?
- Pattern: What type and how distinct?
- Distribution: Even or concentrated?
- Directionality: Visible from multiple angles?
- Dead spots: Areas without colour?
Play of Colour vs Other Phenomena
Distinguishing play of colour from similar effects:
Key Differences
- Play of colour (opal): Diffraction; pure spectral colours
- Labradorescence: Interference; colours in patches
- Iridescence: Thin-film; rainbow sequence
- Adularescence: Interference; single colour glow
Physical Cause
- Opal: 3D sphere array (diffraction)
- Labradorite: Twin lamellae (interference)
- Iridescent gems: Surface layers (interference)
- Moonstone: Exsolution layers (interference)
References
- ↑ 1. Nassau, K. (2001). The Physics and Chemistry of Color (2nd ed.). Wiley-Interscience. ISBN: 978-0-471-39106-7.
- ↑ 2. Read, P. (2014). Gemmology (3rd ed.). Butterworth-Heinemann/Routledge. DOI: 10.4324/9780080507224.
- ↑ 3. Schumann, W. (2009). Gemstones of the World (4th ed.). Sterling. ISBN: 978-1-4027-6829-3.