Crystal guide
How to Identify Opal
SiO₂·nH₂O
Opal is hydrated amorphous silica. Precious opal shows play-of-color; common opal (potch) does not. Identify precious opal by spectral flashes that move with light, Mohs ~5–6.5, and non-crystal habit.
About Opal
Opal is hydrated amorphous silica. Precious opal shows play-of-color; common opal (potch) does not. Identify precious opal by spectral flashes that move with light, Mohs ~5–6.5, and non-crystal habit.
Opal is filed under crystals in the RockScan directory. Composition: Hydrated silica spheres. It forms via silica-rich sedimentary and volcanic settings. Typical occurrence: Australia, Ethiopia, Mexico, USA. For the broader method, see how to identify rock types.
Opal properties
| Mohs hardness | 5–6.5 |
|---|---|
| Class | crystal |
| Scientific name | SiO₂·nH₂O |
| Color | Body color + spectral play-of-color |
| Luster | Vitreous to waxy |
| Crystal system / texture | Amorphous |
| Composition | Hydrated silica spheres |
| Formation | Silica-rich sedimentary and volcanic settings |
| Occurrence | Australia, Ethiopia, Mexico, USA |
How to identify Opal
- Play-of-color for precious opal.
- Mohs ~5–6.5; amorphous.
- Often in nodules or seams; not hexagonal crystals.
Lookalikes to rule out
- Glass imitation: Imitations may show unnatural glitter; precious opal play-of-color moves in sheets.
- Moonstone: Moonstone adularescence is milky blue sheen; opal play-of-color is spectral flashes.
- Iris agate: Different mechanism; agate banding vs opal’s amorphous silica.
Photo tips for Opal ID
- Move light to capture play-of-color.
- Dark background helps precious opal flashes.
- Note potch (common opal) lacks play-of-color.
Snap one specimen in natural light, then open the result in the online studio or the rock identifier app.
Opal FAQ
How do you identify raw opal?
Look for precious play-of-color or waxy common opal in seams/nodules. Hardness below quartz.
Opal vs glass?
Natural play-of-color structure differs from glitter; density and surface cues help.
Is opal a crystal?
It is amorphous mineraloid — popular in crystal shops; we list it under crystals for photo-ID UX.
How to identify opal
Collectors search “how to identify opal” (and “raw opal”) when play-of-color might be glass, moonstone, or potch without flash. Use this field checklist:
- Play-of-color: precious opal flashes spectral sheets that move with light angle — not glitter flakes.
- Hardness: Mohs ~5–6.5 amorphous hydrated silica — softer than quartz (~7).
- Habit: nodules, seams, or potch; not hexagonal crystal points like clear quartz.
- Common vs precious: potch (common opal) lacks play-of-color — still opal, different trade grade.
- Photo first: move light on a dark background for flashes, then confirm with RockScan.
Opal vs glass vs moonstone vs iris agate
- Glass imitation: Unnatural glitter or foil; precious opal play-of-color moves in spectral sheets within the stone.
- Moonstone: Milky blue adularescence; opal play-of-color is multi-spectral flashes.
- Iris agate: Banded chalcedony diffraction; opal is amorphous hydrated silica without agate banding habit.
Photo tips for opal ID
Move a single light over a dark background to catch play-of-color; note potch without flash. Compare agate, quartz, and clear quartz, then confirm in the online studio or rock identifier app.
Related crystals to compare
Browse all: rocks · minerals · crystals · directory · what rock is this · rock vs mineral vs crystal. Confirm with the online studio or rock identifier app.