Opal
Opal species including precious opal (black, white, crystal, boulder, fire) and common opal with play of colour, patterns, and identification.
Introduction
Opal (SiO₂·nH₂O) is a hydrated silica that produces one of nature's most
spectacular optical phenomena: play of colour. Unlike crystalline gems,
opal is amorphous, composed of orderly stacked silica spheres that
diffract light into spectral colours.
Australia dominates world opal production, producing over 90% of
precious opal.
Mineralogy
Structure
Opal is unique among gems:
- Not crystalline: Amorphous (no crystal structure)
- Composition: Hydrated silica (SiO₂·nH₂O)
- Water content: 3-21% (typically 6-10%)
- Formation: Low-temperature precipitation from silica-rich water
Physical Properties
| Property | Value |
|---|---|
| Hardness | 5.5-6.5 Mohs |
| Specific gravity | 1.98-2.25 |
| Refractive index | 1.37-1.47 |
| Optic character | Singly refractive (amorphous) |
| Lustre | Vitreous to waxy |
| Fracture | Conchoidal |
Play of Colour
Play of colour is opal's defining phenomenon—flashing spectral colours
that shift as the stone moves.
Cause
- Uniform silica spheres (150-400nm diameter)
- Regularly stacked in three-dimensional array
- Light diffracts between sphere layers
- Different sphere sizes produce different colours
Larger spheres (>350nm) produce red; smaller spheres produce blue/violet.
Regularity of stacking determines brightness.
Precious vs Common Opal
- Precious opal: Shows play of colour; regular sphere arrangement
- Common opal (potch): No play of colour; irregular spheres
- Fire opal: Valued for body colour; play of colour optional
Opal Types by Body Tone
Black Opal
The most valuable opal type:
- Body tone: Dark grey to black
- Effect: Colours appear most vivid against dark background
- Source: Lightning Ridge, Australia (premium)
- Value: Highest among opal types
White Opal
- Body tone: White to light grey
- Effect: Softer colour display
- Sources: Coober Pedy (Australia), Mintabie
- Value: Lower than black; still valuable if fine
Crystal Opal
- Character: Transparent to semi-transparent
- Effect: Play of colour visible from both sides
- Body: Clear to slightly milky
- Value: High for good specimens
Boulder Opal
- Character: Opal in ironstone matrix
- Source: Queensland, Australia
- Advantage: Matrix provides stability
- Value: Variable; fine material highly valued
Fire Opal
- Body colour: Yellow, orange, red
- Play of colour: Optional; not required
- Source: Mexico (primary)
- Value: Based on body colour intensity
Pattern Types
| 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 |
| Straw/grass | Thin parallel lines | Lower |
Harlequin Pattern
Major Sources
| Source | Type Produced | Characteristics |
|---|---|---|
| Lightning Ridge (Australia) | Black opal | Finest black opal; premium source |
| Coober Pedy (Australia) | White opal | Major producer; good quality |
| Mintabie (Australia) | Black and crystal | Good quality material |
| Queensland (Australia) | Boulder opal | Ironstone matrix material |
| Mexico | Fire opal | Orange to red body colour |
| Ethiopia | Welo opal | Hydrophane type; emerging source |
| Brazil | Various | Limited production |
Ethiopian (Welo) Opal
Treatments
Opal undergoes various treatments:
- Smoke treatment: Darkens body tone (Ethiopian especially)
- Sugar/acid: Historic treatment to darken
- Plastic impregnation: Stabilises; must be disclosed
- Doublets/triplets: Composite stones (not treatment per se)
- Oiling: Temporary enhancement
Care and Stability
Identification Summary
Key features for opal identification:
- RI: 1.37-1.47 (low; single reading)
- SG: 1.98-2.25 (low)
- Play of colour: Diagnostic for precious opal
- Amorphous: SR; no crystal structure
- Water content: Can affect stability