Opal
SiO₂·nH₂O · Amorphous · also called precious opal, common opal, potch
Opal is hydrated silica gel that never crystallised. In precious opal, silica spheres are stacked regularly enough to diffract light, producing the famous play of colour. Most opal, however, is 'common opal' or potch, with no colour play at all.
Properties
- Mohs hardness
- 5.5–6.5
- Streak
- white
- Lustre
- vitreous, waxy, dull
- Colour
- Body colour ranges from milky white to black. Precious opal adds flashes of spectral colour that shift as you tilt it.
- Cleavage
- No cleavage.
- Fracture
- Conchoidal
- Specific gravity
- 1.9–2.3 (water = 1)
- Magnetism
- Not magnetic
- Acid test
- No reaction
- UV
- Some specimens fluoresce
Tests that confirm opal
- Noticeably light in the hand — opal contains water and is less dense than quartz.
- Play-of-colour flashes change with viewing angle, unlike surface iridescence.
- Softer than quartz and can craze or crack if it dries out too fast.
What opal gets confused with
Each of these is settled by one test. Run that test first.
Chalcedony is denser and harder, with no play of colour.
Imitations show colour patterns that repeat regularly or sit only on the surface.
Where it is found
Weathered volcanic rock and sedimentary basins; Australia produces the majority of precious opal, with Ethiopia and Mexico also significant.
Is it worth anything?
Black opal with strong play of colour is very valuable. Common opal is inexpensive. Doublets and triplets are thin slices glued to a backing and are worth far less than solid stones.