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 hydrated amorphous silica (no crystal system) containing 3–21% water by weight (typically 6–10%), with hardness 5.5–6.5 Mohs, SG 1.98–2.25, and RI 1.37–1.47 (spot, singly refractive). [1] Play of colour, opal's defining feature, is produced when uniformly sized silica spheres (150–400 nm diameter) stack in regular three-dimensional arrays and diffract visible light; larger spheres produce red, smaller spheres produce blue and violet. [2] Australia supplies over 90% of precious opal; Lightning Ridge, New South Wales, produces black opal (the most valuable type), where a dark body tone maximises colour display. A fine Lightning Ridge black opal with harlequin pattern (large angular mosaic patches) can sell for hundreds of thousands of dollars. Ethiopian Welo opal is often hydrophane (absorbs water, becomes transparent when wet), requires care in testing, and trades below comparable Australian material. The water content makes all opals sensitive to dehydration and temperature shock. [2]
Mineralogy
Structure
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) [2]
- 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. [2] Regularity of stacking determines brightness.
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) [1]
- 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
References
- ↑ 1. Read, P. (2008). Gemmology (3rd ed.). Butterworth-Heinemann. ISBN: 978-0-7506-6449-3. DOI: 10.4324/9780080507224.
- ↑ 2. Zhao, S.; Bai, J. (2020). Crystallinity and Play-of-Colour in Gem Opal with Digit Patterns from Wegel Tena, Ethiopia. Minerals, 10(7), 625. DOI: 10.3390/min10070625.