Identification guide · Semi-Precious Stones
How to Identify Opal
Opal is usually white, black, and fire with a vitreous luster and is translucent. It has a Mohs hardness of 5.5. It is about as hard as window glass: a knife barely marks it, while a steel file (Mohs 6.5) will. Its streak is white. It is most often confused with Moonstone, Chalcedony, and Slag Glass; the tests below separate them.
Opal identification checklist
| Color | White, Black, Fire, Blue, Pink | Look at a clean, dry surface in daylight. Color zoning, banding or spots are as diagnostic as the hue. |
|---|---|---|
| Luster | Vitreous | Fresh surfaces shine like glass; a dull surface usually means weathering, so chip or wet a corner. |
| Transparency | Translucent | Hold a thin edge up to a bright light and note whether light passes through. |
| Mohs hardness | 5.5 | It is about as hard as window glass: a knife barely marks it, while a steel file (Mohs 6.5) will. |
| Streak | White | Drag a corner across unglazed porcelain (the back of a bathroom tile) and check the powder color. |
| Cleavage | None | |
| Fracture | Conchoidal | Broken surfaces show smooth, curved, shell-like scoops. |
| Specific gravity | 2.1 | Noticeably light in the hand for its size (specific gravity about 2.1). |
| Crystal system | Amorphous |
Extra tests for Opal
Glass check. Opal shares a glassy luster and conchoidal fracture with man-made glass. Glass sits near Mohs 5.5 and often contains round gas bubbles, so a hardness test and a hand lens separate them.
Opal look-alikes and how to tell them apart
Both sit near Mohs 5.5, so a scratch test alone will not separate them. Lean on luster, streak and heft instead. Opal vs Moonstone →
Chalcedony is harder (Mohs 6.5–7) than Opal (Mohs 5.5), so Chalcedony will scratch Opal and not the other way round. It is about as hard as window glass: a knife barely marks it, while a steel file (Mohs 6.5) will. Opal vs Chalcedony →
Both sit near Mohs 5.5, so a scratch test alone will not separate them. Lean on luster, streak and heft instead. Opal vs Slag Glass →
Opal is harder (Mohs 5.5) than Smithsonite (Mohs 4.5), so Opal will scratch Smithsonite and not the other way round. A steel knife blade (Mohs 5.5) scratches it; a copper coin does not. Opal vs Smithsonite →
Is my Opal real?
- Hardness: real Opal sits at Mohs 5.5. Glass imitations are about 5.5, so use a steel file and a known reference mineral rather than glass alone.
- Bubbles: round gas bubbles under a 10x loupe mean glass or resin, never a natural stone.
- Color: dyed material concentrates color in cracks and pores; look for pigment collecting at the edges of fractures.
- Temperature: natural crystals feel cool and take longer to warm in your hand than plastic or resin.
Field tips
- Check whether the specimen's color and texture match typical Opal examples.
- Use hardness, streak, luster, grain size, and locality together; no single trait is enough for every specimen.
Opal identification FAQ
How can you tell if a rock is Opal?
Check the color (white, black, fire, and blue), the vitreous luster, a Mohs hardness of 5.5 and a white streak. Photographing it with the StoneSnap app gives a match in seconds.
What is the hardness of Opal?
Opal has a Mohs hardness of 5.5. It is about as hard as window glass: a knife barely marks it, while a steel file (Mohs 6.5) will.
What is the difference between Opal and Moonstone?
Opal (Mohs 5.5, vitreous luster) and Moonstone (Mohs 6, vitreous luster) look alike, but hardness, streak and heft usually separate them. See the full Opal vs Moonstone comparison for a side-by-side table.
How do I know if my Opal is real?
Real Opal matches the hardness listed above, shows no round gas bubbles under magnification, and does not concentrate dye in cracks. Glass and resin fakes fail at least one of those checks.
Is Opal worth anything?
Entry-level Opal specimens sell for around $50, typical collector pieces about $500, and premium examples $10000 or more depending on size, quality and locality.
What color is Opal?
Opal is most often white, black, fire, and blue. Color varies with impurities and locality, so confirm with hardness and luster rather than color alone.
