Formation
|
Opal is deposited in cavities and
fissures in rocks from low temperature, silica-bearing water. Opal may
also replace existing structures, such as
fossils or soluble minerals. |
Composition
|
Opal is composed of amorphous, hydrated silica: SiO2
· n H2O
While opal usually contains around 6 - 10% water, it may contain as much
as 25% |
Structure
|
Precious opal consists of regular, three-dimensional
arrays of tiny, uniformly sized silica spheres. The spheres show a
shell-like build-up with a nucleus in cross section and must have a diameter of 200 - 350 nm
to be able to produce a play-of-colour.
|
Mohs hardness
|
Ranges from 5.5 - 6.5 |
Specific gravity
|
2.1 / Fire Opal is slightly lower at 2.0 |
Refractive index
|
Ranges from 1.44 - 1.46 / Fire Opal may have an RI as
low as 1.40
Opal is near-amorphous and therefore singly refractive. |
Transparency
|
Opaque / translucent (Semi-crystal Opal)
/ transparent
(Crystal Opal) |
Optical effects
|
Play-of-colour (Precious Opal) / Chatoyancy (Cat's eye
Opal) |
Luminescence
|
Most opals show a whitish fluorescence under LW and SW
UV light which is used for example to spot rough opal in noodling machines.
Also opal usually shows some phosphorescence. This may be used to
distinguish natural opal and "synthetic" opal (e.g. Gilson and
Kyocera
Opal)
which commonly does not exhibit phosphorescence. |
