 Microscopical
studies of Opal
yield that it consists of very small
spheres of crystoballite (silicone-dioxide), embedded in amorphous
solidified silicone-gel. Due to the interference of light on these spheres,
a certain effect can be recogniced: opal shows all colours of the spectrum.
Watched under different angles, the spots change their colour.
 According
to the basic colour of opals, one can distinguish between dark
opal (dark
basic colour), white Opal
(white basic colour) and fire
opal (red and translucent or transparent).
Jelly-opal
is an ordinary opal that doesn´t
show any light effects and is not translucent. Water-opal
is a translucent but dull opal.
 Opal
is chemically counted among the quarz group, but due to its amorphous structure
it is a unique gem. Its hardness is about 5,5-6.
Occurances of opal: Hungary (depleted today),
Brazil, Australia.
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