Play of Colour
Play of colour in precious opal including cause, pattern types, body tones, and quality assessment.
Introduction
Play of colour is the flashing spectral colours seen in precious opal. It is
opal's defining characteristic and distinguishes precious opal from common
opal (potch). The effect results from light diffraction by regularly arranged
silica spheres within the opal structure.
Unlike interference effects (adularescence, labradorescence), play of colour
is a diffraction phenomenon, producing pure spectral colours that shift
with viewing angle.
Cause
The physics of play of colour:
Silica Sphere Structure
- Opal is composed of tiny silica spheres (SiO₂)
- Spheres must be uniform in size (150-400nm)
- Spheres must be regularly arranged in 3D array
- Acts as a diffraction grating for light
Colour Determination
- Sphere size determines colour: Larger spheres = longer wavelengths
- Spheres >350nm: Produce red (most valuable)
- Spheres ~250nm: Produce green
- Spheres ~150nm: Produce blue/violet
- Regularity of arrangement determines brightness
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)