COMPARE GUIDE
Black OpalVSBoulder Opal
Side-by-side comparison of physical properties, fluorescence, and crystal system — to identify by observation.
Physical propertiesPROPERTIES
| Property | A: Black Opal | B: Boulder Opal | Verdict |
|---|---|---|---|
| Refractive index (RI) | 1.430–1.470 | 1.430–1.470 | Nearly equal |
| Specific gravity (SG) | 1.97–2.20 | 2.10–2.50 | Overlap |
| Mohs hardness | 5.5–6.5 | 5.5–6.5 | Nearly equal |
| Birefringence | — | — | — |
| Dispersion | — | — | — |
Optics & fluorescenceOPTICS & UV
| Property | A: Black Opal | B: Boulder 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) |
ObservationOBSERVATION
◆Black opal has a uniformly dark opal body throughout; boulder opal shows the actual ironstone host rock as visible matrix behind or alongside the opal seam.
Color categoriesCATEGORIES
A: Black Opal
Opal
B: Boulder Opal
Opal
DISTINGUISHING TOOLS · 2 ITEMS
Distinguishing tools
Tools that decide between Black Opal and Boulder 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.