Black Opal
Last updated: April 2026
Black opal from Lightning Ridge, NSW, Australia is commercially the most valuable opal variety. It is distinguished from Ethiopian Welo opal by its non-hydrophane character — it does NOT absorb water or change transparency when wet. To test: breathe on the stone or briefly immerse; if play-of-color disappears and returns as it dries, it is hydrophane (Ethiopian type). Black opal's body tone is provided by thin potch (common opal) layers between the silica spheres. Doublets and triplets are common imitations — examine the girdle profile for a flat base junction.
Black Opal has a specific gravity of 2.10 and a refractive index of 1.440–1.460, with a Mohs hardness of 5.5–6.5. It is singly refractive.
Physical & Optical Properties
Measured with a polariscope, a refractometer, a hydrostatic balance, a Chelsea filter, a UV torch, a loupe, a darkfield loupe and a spectroscope — step-by-step for each.
Through the refractometer
Drawn from the refractive index above, on a 1.40–1.81 scale. How to take this reading.
Through the polariscope
Drawn from the optic character and transparency above. A polariscope separates singly refractive, doubly refractive and aggregate; the optic sign needs a conoscope. How to take this reading.
Specific gravity, and what it rules out
Drawn from the specific-gravity range above, against every other species in the table. How to take this reading.
Through the dichroscope
Drawn from the pleochroism above. Two windows that match mean either no pleochroism or the wrong angle — turn the instrument to tell. How the dichroscope compares.
Related: Opal Varieties
Key Differentiators
- Dark body tone (N1–N4 on GIA tone scale) with vivid play-of-color
- Non-hydrophane — does NOT absorb water (unlike Ethiopian Welo opal)
- Higher SG ~2.10 than white/crystal opal (~1.95–2.00)
- Lightning Ridge, New South Wales, Australia provenance
- Potch (dark common opal) base often visible at girdle or base
Natural vs. Synthetic
Synthetic black opal is commercially available (gilson_process). Distinguishing natural from synthetic typically requires microscopic examination of internal features.
Loupewise Pro includes a two-phase natural vs. synthetic testing protocol for Black Opal.
Common Simulants
- Opal Doublet / Triplet: Assembled doublets and triplets use a thin natural opal layer over a dark backing to simulate black opal color. Profile examination under magnification reveals the junction line between layers; immersion in water may de-bond older doublets.
- Black Spinel: Opaque black, isotropic. No play-of-color — play-of-color is the definitive test. SG 3.58–3.61 (much heavier than black opal ~2.15); refractometer gives ~1.718 on flat surface vs opal ~1.37–1.47.
- Obsidian: Volcanic glass, opaque black. No play-of-color; conchoidal fracture; lower SG ~2.4 vs opal ~2.15 (similar range but obsidian lacks play). Inclusions under magnification show flow lines and no silica sphere structure.
Treatments
- Smoke treatment (darkens body tone — detectable by hot point or immersion)
- Sugar-acid treatment (carbonization darkens body — detectable by microscopy)
Measurement Guides
Identifying a black opal? Loupewise walks through these tests in order — RI, SG, fluorescence, and more.
Try Loupewise Free →Studying for the GIA, FGA, or FEEG practical? Black Opal is one of the 132 species in Loupewise’s drill and exam pool — practice identifying it from its measured properties under timed exam conditions.
Practice exams →