* opal gemstone
Opal: identification, value, and how to spot a fake
Hydrated silica with characteristic play-of-color (precious opal) from light diffraction off ordered silica spheres. Australian black opal (Lightning Ridge) is the most valuable. Common opal lacks play-of-color. Sensitive to dehydration and impact.
Published May 30, 2026
Quick facts
- Mineral family
- opal
- Chemical formula
- SiO2·nH2O
- Mohs hardness
- 5.5–6.5
- Refractive index
- 1.45–1.46
- Specific gravity
- 1.98–2.25
- Crystal system
- amorphous
- Luster
- vitreous, waxy
- Birthstone month
- October
Colors
Opal is found in white, black, fire orange, boulder (with matrix), crystal, all colors via play-of-color. The body color, its saturation, and how evenly it spreads through the stone are among the strongest drivers of value.
How to tell real from imitation
No single test settles it, but a few grounded checks quickly narrow things down for Opal. Hardness, brilliance, and the list of stones it is commonly confused with all give you something concrete to look for.
- Scratch resistance: genuine Opal sits at Mohs 5.5–6.5, so it should resist scratching from softer materials — a stone that mars far too easily is suspect.
- Brilliance & luster: Opal shows a vitreous, waxy luster and a refractive index of 1.45–1.46; imitations often look glassier or flatter under the same light.
- Density: a real Opal has a specific gravity near 1.98–2.25, so it feels heavier or lighter in the hand than many look-alikes of the same size.
- Commonly imitated by: opalescent glass, synthetic opal (Gilson). If a deal looks too good, suspect one of these substitutes first.
- Fast first read: photograph the stone in the Jewelry Identifier app to estimate the gemstone and its likely value range, then confirm anything important with a gemological lab.
Common treatments
- smoke treatment (Andamooka)
- oiling
- polymer impregnation
Treatments are routine for many Opal stones, but they materially affect value and should always be disclosed — an untreated, lab-certified stone commands a clear premium over a treated one.
What to know about Opal
Hydrated silica with characteristic play-of-color (precious opal) from light diffraction off ordered silica spheres. Australian black opal (Lightning Ridge) is the most valuable. Common opal lacks play-of-color. Sensitive to dehydration and impact.
Sources
* Frequently asked
FAQ
- Q. Is Opal hard enough for everyday rings?
- A. Not ideally. At a Mohs hardness of 5.5–6.5, Opal is soft enough to scratch with everyday wear, so it is better suited to earrings, pendants, or occasional-wear rings with protective settings.
- Q. How can I tell Opal from opalescent glass?
- A. Opal and opalescent glass can look alike to the naked eye, but they differ in measurable properties — refractive index (1.45–1.46 for Opal), specific gravity (1.98–2.25), and hardness (Mohs 5.5–6.5). A gemologist confirms the distinction with a refractometer and loupe; the Jewelry Identifier app gives a fast first read from a photo.
- Q. Does Opal come in colors other than white?
- A. Yes. Opal occurs in white, black, fire orange, boulder (with matrix), crystal, all colors via play-of-color. Color is driven by trace elements and treatment history, and it is one of the biggest factors in a stone's value.
* Try it
Identify the jewelry in your hand, right now.
Just take a photo — AI reads the metal, gemstone, hallmark, era, and an estimated value range in seconds. First two scans free, no account required.
Download on the App Store* Related