At
opal info we provide all information on all types of
opals including Boulder Opal, Black Opal, White and
Crystal Opals.
The opal information and descriptions following are
quoted or paraphrased mainly from A Field Guide to Australian
Opals by Barrie O’Leary (Rigby Ltd., Australia,
1977).
What is an Opal?
Opal is hydrated silica (SiO2.nH2O). It is formed of
minute particles of silica in tight spherical aggregates.
For a more detailed definition, and to find out
how
and when opal was formed, click here. Opal is
a true precious stone, not a semi-precious one, as many
believe. There are, however, ‘common’, or
non-precious forms. Common opal is found throughout
the world, but precious opal is found overwhelmingly
in Australia, with only 5% being found elsewhere, mainly
in Mexico. One form of common opal is potch, of which
there are many varieties and colours. Transparent amber
potch is called fire opal. When it’s very transparent
and bright yellow in colour, it is called a sun opal.
Black potch is often used as the backing in opal doublets
and triplets. To learn about
doublets
and triplets, click here. Ninety-five percent
of all opal mined is potch. Ninety-five percent of the
remaining opal is fairly mediocre. Thus, only five percent
of five percent of opal mined can truly be called precious
opal. Precious opal usually only refers to solid opals,
but Queensland boulder opal is often thick enough and
of high enough quality to qualify. A 'true' opal is
one in which a play of colour can be seen from any angle.
One of the main types having this characteristic is
the harlequin opal, and its many subtypes. Harlequin
opals are considered to be the most valuable of all
opals. Click here to learn about
harlequin
opals and how to recognise them.
Not every opal has a harlequin pattern, of course. Not
only that, but opal is incredibly changeable, with the
colour, and even pattern, changing from different angles.
Opals with a perfect pattern are also not common, with
most being made up of several forms of pattern. Click
here to learn about other
types
of opal patterns.
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