COMPARE GUIDE
Black OpalVSMatrix Opal
Side-by-side comparison of physical properties, fluorescence, and crystal system — to identify by observation.
| Property | A: Black Opal | B: Matrix Opal | Verdict |
|---|---|---|---|
| Refractive index (RI) | 1.430–1.470 | 1.430–1.470 | Nearly equal |
| Specific gravity (SG) | 1.97–2.20 | 2.00–2.50 | Overlap |
| Mohs hardness | 5.5–6.5 | 5.5–6.5 | Nearly equal |
| Birefringence | — | — | — |
| Dispersion | — | — | — |
| Property | A: Black Opal | B: Matrix Opal |
|---|---|---|
| Crystal system | 非晶質（含水シリカ） | 非晶質（含水シリカ） |
| Pleochroism | — | — |
| Chelsea filter | inert | — |
| UV (long-wave) | weak (green / blue) | — |
| UV (short-wave) | inert | weak (white) |
◆Black opal (Lightning Ridge and elsewhere) is solid precious opal with naturally dark body color from included carbon, manganese oxide, or chrome compounds throughout the opal itself — not a composite of opal plus dark host. SG 2.10 (consistent), no visible host-rock grain structure at the surface, and continuous broad play-of-color across the full surface distinguish solid black opal from sugar-treated Andamooka matrix. The Andamooka treatment can produce a superficially similar dark ground but always shows host-rock grain structure under 10× examination, and the SG runs lower (2.0–2.4 typical) than solid black opal.
Distinguishing tools
Tools that decide between Black Opal and Matrix Opal in practice.
Note: this page is auto-generated from published GIA / CGL reference values. Individual stones, treatments, and synthetics may fall outside these ranges. For a definitive identification, consult an accredited gem laboratory.