Opal characteristics Opal classification Types of opal Varieties of opal Body tone Transparency Play-of-colour Brightness Pattern Shape and face Origin


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Opal Lexicon

Opal characteristics

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.

 

 

 

 

 

 

 

 

 

 

 


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