Precious Opal Identification Guide
How to identify precious opal by its diagnostic play-of-color, hydrated silica composition, low hardness and density, and how to separate it from common opal and imitations.
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What Precious Opal Looks Like
Precious opal is hydrated amorphous silica famous for its play-of-color — flashing patches of spectral color (red, orange, green, blue, violet) that shift as the stone moves. The body color may be white, gray, black, blue, or transparent, with the color flashes floating within or across it. Luster is vitreous to resinous or waxy, and the stone ranges from transparent to opaque. The shifting rainbow flashes (not surface iridescence) are the defining trait.
Step-by-Step Field ID Checklist
- Observe play-of-color. Rotate the stone under a single light source; genuine precious opal shows patches of pure spectral color that move and change.
- Distinguish from body color. The flashing colors should appear to come from within, separate from the base hue.
- Test hardness gently. Opal is soft (5.5–6.5) and scratches easily — handle carefully.
- Check the feel and weight. It is notably light (low density) and may feel slightly warm.
- Look for a curved fracture. Conchoidal fracture, no cleavage.
Key Diagnostic Tests
- Mohs hardness: 5.5 to 6.5 — softer than quartz; will not scratch glass reliably.
- Cleavage: None; conchoidal fracture.
- Streak: White.
- Specific gravity: Low, about 1.9 to 2.3 (lighter than most gems).
- Water content: 3–21% water; hydrophane varieties can absorb water and change appearance.
- Acid: No fizz to HCl.
Common Look-Alikes and How to Tell Them Apart
- Common opal (potch): Has the same composition but no play-of-color; only precious opal flashes spectral colors.
- Labradorite/spectrolite: Shows schiller (labradorescence) but is harder (6–6.5), has cleavage, and the sheen is more metallic sheets than floating color patches.
- Imitation opal (Gilson, opalite glass, plastic): Synthetic opal often shows a regular columnar "lizard-skin" or "snakeskin" color pattern and a too-perfect look; opalite glass is just milky with surface glow, no true play.
- Doublets/triplets (assembled): A thin opal layer cemented to dark backing and a quartz cap; check the side profile for a flat glue line and dome.
- Fire agate/iris agate: Iridescence is from thin films, harder (7), and waxy.
Where Precious Opal Is Found
Most of the world's precious opal comes from Australia (Lightning Ridge for black opal, Coober Pedy and Andamooka for white/light opal, Queensland for boulder opal). Important sources also include Ethiopia (Welo), Mexico, Brazil, and the western United States (Nevada, Idaho, Oregon). It forms from silica-rich solutions filling cavities and seams in sedimentary and volcanic rocks.
Frequently asked questions
How can you tell if it's real precious opal?
Genuine precious opal shows true play-of-color, flashing patches of spectral color that shift and move from within as you rotate it, plus low hardness (5.5 to 6.5), low density, and conchoidal fracture. Imitations show regular columnar color patterns or mere surface glow.
What does precious opal look like?
It is hydrated silica with a white, gray, black, blue, or clear body that displays flashing rainbow patches of red, green, blue, and violet that change with viewing angle.
Precious opal vs common opal: what is the difference?
Both are hydrated silica, but precious opal shows play-of-color (spectral flashes), while common opal (potch) lacks it and appears as a solid or milky color.
How do you tell real opal from a doublet or triplet?
Examine the stone from the side: a doublet has a flat glue line where a thin opal slice meets a dark backing, and a triplet adds a clear domed cap. Solid opal shows color continuing through its thickness.
Precious opal vs labradorite: how do you tell them apart?
Opal shows floating patches of pure spectral color and is soft with no cleavage, while labradorite shows broad metallic sheets of sheen, is harder, and has distinct cleavage planes.