Opal occurs in
a myriad of forms, some of which are obvious
to the naked eye, and others which do not
appear in the least like opal. Most of us
are familiar with precious opal, with its
unique (in the inorganic world of nature)
of colours. The gem material itself
occurs in a variety of patterns, and may
also be found in various forms replacing
small and large marine creatures, bones
and wood. Associated with the gemstone in
its natural occurrence is potch, which,
while showing no play of colour, can occur
in a variety
of shades of light grey to black, with
intermediate smoky, and occasional yellow
or orange colours.
Many of these not only make attractive specimens
for the collector, but can be cut and polished
to make acceptable ornamental gem material.
Most of these materials are composed of
the amorphous form called opal-A. However
some of the volcanic opals are composed
of more crystalline material, opal-CT or
Another form of opal-A is that known as
It most commonly occurs associated with
volcanic rocks, usually deposited in vugs
as a transparent colourless glass-like
material. It is sometimes found in this
form in upper zones of ore deposits. A further
form of opal-A is found in some volcanic
areas where there is (or has been) much
thermal activity at the earth's surface.
Large deposits of silica, known as siliceous
sinter, or geyserite, may be formed.
These may be found in areas such as Yellowstone
National Park in the U.S.A., but perhaps
the most famous example was the so-called
'pink terraces' at Rotorua in New Zealand.
These terraces were destroyed by an earthquake
Common opal, usually opal-CT, also occurs
in a wide variety of forms and body colour,
but does not show the play of colours found
in precious opal. The reason for this is
the fundamentally different microstructure
- but more of that later.
The body colour of common opal is largely
due to impurity elements, particularly iron.
Many opals contain a substantial amount
of iron oxide; one sample was found to contain
more than 20% ferric oxide. These opals
range in colour from pale to dark brown,
rarely reddish, depending on the amount
and nature of the iron oxide impurity.
Opals with impurities such as alumina, lime,
or alkalies usually contain very little
iron, and tend to be pale grey to white
in colour. Other impurities, such as traces
of copper or nickel, for example, may give
them a green tinge. If the amount of impurity
is small and the crystallites very small,
common opal may be translucent and virtually
Many common opals vary
in colour and pattern due to uneven distribution
of impurities, and some of these also become
useful ornamental stones. Particularly attractive
are those containing dendrites
usually of manganese oxide. The resultant
fern-like growths can give striking cabochons
for jewellery. Others, as with some types
of agate, can give rise to landscape and
Common opal also may replace organic materials,
especially wood. It can preserve the woody
structures in great detail. It is sometimes
associated with chalcedony in this form.
All of the above can readily be recognised
in a hand specimen as a form of opal. However
there are numerous forms of opal which are
less easily recognised. Many of these are
derived from the skeletons of plants or
Phytoliths are small, often needle-like
remains derived from grasses and other plants
and are to be found in soils and may sometimes
accumulate in small deposits. Many trees
incorporate silica into their cellular structures;
the opaline materials eventually return
to the soil on the death of the tree and
decomposition of the wood.
Microscopic marine animals such as diatoms
and radiolaria can have skeletons which
are composed of silica. On the death of
these creatures vast deposits of earthy
opaline materials can accumulate.
Thus, we find soft or firm earthy deposits
of diatomaceous earth, often mixed with
clay; the latter is termed opal claystone,
tripoli, gaize and opoka in various countries
in which they are found. In this form it
is an important industrial raw material.
In the deeper parts of the ocean, these
skeletons are still building up deposits,
and undergoing changes which result in changes
in their microstructure,
but again, more of this later.