Blue Opal Identification Guide
Identify blue opal by its waxy-to-resinous luster, hardness 5.5–6.5, conchoidal fracture, and milky-to-translucent blue body, versus harder blue stones.
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What Blue Opal Looks Like
Blue opal is a hydrated silica (SiO2·nH2O) that is amorphous (non-crystalline). Most blue opal is 'common opal' (no play-of-color), showing a milky to translucent body in shades from pale sky-blue to greenish-blue, often clouded or mottled. Luster ranges from waxy to resinous or sub-vitreous. Some blue opal (e.g., Peruvian) is colored by copper. It is massive, with no crystal form, and may show a soft internal glow.
Step-by-Step Field ID Checklist
- Body color and clarity: Soft milky blue to blue-green, translucent to opaque, often with white or brown matrix.
- Luster: Waxy to resinous — a key opal trait.
- No crystals: Amorphous, smooth-surfaced.
- Hardness test: A steel knife scratches it with effort; it will not scratch glass cleanly (Mohs 5.5–6.5) — softer than chalcedony.
- Fracture: Conchoidal.
- Acid test: No fizz.
Key Diagnostic Tests
- Hardness: 5.5–6.5. Softer than chalcedony/agate (7) — the main separator from blue chalcedony.
- Streak: White.
- Fracture: Conchoidal, no cleavage.
- Acid reaction: None.
- Specific gravity: ~1.9–2.2 — noticeably lighter than chalcedony (~2.6); blue opal feels light for its size.
- Water content: Hydrophane types can stick to the tongue and absorb water.
Common Look-Alikes and How to Tell Them Apart
- Blue chalcedony: Harder (7, scratches glass) and denser (SG ~2.6); blue opal is softer and feels lighter.
- Larimar: Harder (4.5–5 but with distinct red-brown matrix and turquoise web); larimar is pectolite, not silica.
- Blue calcite: Soft (3) and fizzes in acid; opal does not fizz.
- Opalite (man-made glass): Sold as 'blue opal'; opalite is manufactured opalescent glass showing orange flash on a milky body, with bubbles and uniform color.
- Chrysocolla/gem silica: Copper-blue but often harder when silicified; gem silica is chalcedony (7).
Where Blue Opal Is Found
Opal forms from low-temperature silica-bearing solutions percolating through rock, depositing in cavities, seams, and weathered volcanic and sedimentary host. Notable blue common opal comes from Peru (Andean copper-bearing blue opal), Indonesia, Oregon (USA, blue-green 'Owyhee' opal), and various other volcanic regions. Look for it as veins and nodules in altered volcanic rock.
Quick Confidence Check
A waxy-to-resinous, milky blue, amorphous stone that feels light, is scratched by a knife (but does not scratch glass), and does not fizz in acid is blue opal.
Frequently asked questions
How can you tell if it's real blue opal?
Real blue opal is amorphous hydrated silica: waxy-to-resinous luster, hardness 5.5–6.5 (scratched by a knife), conchoidal fracture, no acid fizz, and a low specific gravity (~2) so it feels light for its size.
Blue opal vs blue chalcedony — what's the difference?
Chalcedony is harder (7, scratches glass) and denser, while blue opal is softer (scratched by a knife) and feels lighter. Use a glass-scratch test and heft to tell them apart.
Is blue opal the same as opalite?
No. Opalite is man-made opalescent glass (often showing an orange flash and bubbles), while natural blue opal is hydrated silica with a waxy luster and natural matrix.
What does blue opal look like?
It looks like a soft, milky to translucent sky-blue or blue-green stone with a waxy or resinous glow and no visible crystals, often with patches of white or brown host rock.
Does blue opal have play-of-color?
Usually not. Most blue opal is 'common opal' without the flashing rainbow play-of-color; its appeal is the solid milky blue body color, often from copper in Peruvian material.