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| potch
opal |
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| common
opal |
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| petrified
wood |
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| opalized
wood |
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| hyalite
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THE SILICA
FAMILY |
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Precious
Opal |
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This
is the best known variety of opal. It shows
a 'play
of colours' - i.e. a colour pattern
in which the colours in the individual segments
of the pattern change as the opal is moved
in relation to the light falling on it;
see also section, Presentation
of Opal. Precious opal is unique in
the world of minerals because of the manner
in which the colour is created. The reasons
for the display of colours is dealt with
in the section, The Colour
of Opal.
Precious and 'potch'
opal normally contain between 5% and 8%
water. In the Australian opal fields, opal
showing little or no play of colour is termed
potch. This material may range from transparent
reddish or brownish to plain grey or black
in colour. The origin of the term potch
is obscure. O'Leary (R0891)
suggests that it may be a corruption of
'potsherd', meaning a piece of broken pottery.
It is interesting to note that Curran, in
1896 (R1606),
spelt the term 'potsh'. Both precious opal
and potch are composed of minute spheres
of silica; details of this structure are
given in the section, The Colour of Opal.
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Other
Opal |
In the following sections
are brief descriptions of other types of
opal, usually put together under the term
'common opal', but which may also include
other related types. They do not show a
true play of colour, and are commonly semi-glassy
in nature.
The
other types of opal include:
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Common
opal |
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Common opal
occurs under a wide variety of conditions,
and often contains impurities such
as iron which contribute to its colours
and patterns. The water content is
similar to that of precious opal and
potch. Different varieties have been
given numerous names, mainly dating
back to the 19th century. These names
were largely based upon appearance,
and at least in some cases, lack of
knowledge of the nature of the material.
They may be considered as sub-varieties
of common opal.
Some terms which
come into this category are:
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Cachalong:
Opaque, some almost like porcelain |
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Girasol:
Translucent, bluish white |
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Menilite:
Greyish, concretionary,
with high magnesium content |
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Jasp-opal:
Red, brown to yellow in colour,
containing iron oxide |
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Hydrophane:
Porous opal which becomes transparent
on immersion in water |
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Fire
opal: Transparent to
translucent, reddish to orange
in colour; may appear similar
to the fire opal from Mexico,
but differs in properties. |
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Opalite:
A term used by lapidarists
in Australia, applied to common
opal with a glassy or conchoidal
fracture. It is commonly opaque,
but may be translucent, or even
transparent. |
Some of these
varieties are illustrated. |
Wood
Opal
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nature, wood may be replaced by various
materials to form 'petrified wood'.
The commonest material to replace
wood is silica of one type or another.
Most frequently the replacing silica
is in the form of chalcedony,
but sometimes it is opal.
Occasionally both forms of silica
may play a part. The most perfect
preservation of the fine structures
within the cell walls of the plants
occurs, however, when the replacement
mineral is opal. |
| Hyalite
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| This
is a special type of opal which is
often classified with common opal.
Hyalite is, however, a very distinctive
material. It is always colourless
and glass clear, and contains less
water than other forms of opal. Its
water content is usually of the order
of 3-4%. Hyalite is most commonly
found in association with volcanic
rocks, especially in small cavities,
or vugs. It may also be found coating
secondary minerals in the weathering
zone of ore deposits. |
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