for opal has always been an uncertain activity.
Most of the known deposits have been found
because of the occurrence of small pieces
of weathered opal on the ground surface,
indicating the probable presence of the
gem nearby. Because of the extreme
dryness of the climate in the Australian
opal fields and the lack of regular runoff
of water, the opal weathering out has tended
to stay on the surface near the matrix sediments
from which it has weathered. This is in
contrast with the Brazilian deposits, where
higher rainfall and more regular streams
have resulted in some opal being recovered
from alluvial deposits.
In most cases, opal has been found by local
residents of the areas; not by the professional
prospector. Once the first deposits have
been exploited, the prospector and miner
then fan out from the initial discovery
area, sinking shafts in the hope of cutting
an opal horizon. This has been the history
of all of the major opal fields of Australia.
Once the initial areas of discovery have
been pegged out and tested, with some claims
being profitable, others barren, prospectors
may sink shafts in nearby areas. Many extensions
of the original fields, have been discovered
in this way, especially in the Lightning
Ridge and Coober Pedy areas.
as to where to sink a shaft is the most
difficult as there are usually few direct
surface indications. However, the experienced
miner has been able to improve the odds
by his knowledge of surface indicators.
Aracic, in his book 'Discover Opals' (R1774)
describes how he does this, after relying
on the type of plant growth to define sub-surface
It is well known that the main opal fields
occur around the edge of the Great
Artesian Basin, so that this zone, an
enormous area in itself, is the logical
area for exploration. There have been numerous
minor discoveries of fields which have yielded
a little commercial production, especially
in South Australia,. In general, these small
fields have been located from surface exposures.
Systematic large scale exploration for opal
in completely new areas has seldom been
undertaken because of the highly irregular
distribution of the mineral. The South Australian
Mines Department has undertaken exploration
at Andamooka and Coober Pedy, and a few
companies have been formed from time to
time to systematically explore for new deposits.
Geologists have been employed to study the
geology and the nature of occurrence of
the opal to develop a more systematic programme
of exploration. However, the work still
tends to be carried out at the margins of
is usually carried out by a drilling program
using calyx drills of about 30 cm diameter.
The shafts are put down to the opal level,
which can usually be recognised by the occurrence
of potch, hard bands, or other changes in
lithology. Because of the speed with which
the holes can be drilled, together with
the technique of spreading the base of the
holes at the opal level, it is not uncommon
for small amounts of opal to be recovered
in the neighbourhood of known fields, thus
helping to finance the exploration.
The earliest opal deposits to be worked
in Australia were close to the surface,
but as these were mined out, it was realised
that the 'opal levels' could be found at
greater depths. Shafts were usually rectangular
in shape and about one metre by 0.7 metre.
These were laboriously dug by hand to depths
of up to 30 metres and ladders, or merely
ropes, were used to climb in and out of
the mines. Two or three men usually mined
each lease, with the surface man using a
windlass to haul buckets of barren or
opal-bearing ore to the surface. This mullock
was dumped around the mouth of the shaft,
which was timbered above ground level, resulting
in the patterns of small, white 'craters'
so strikingly seen from the air.
The shafts were sunk to about 1.5 metres
below the opal level. These levels could
be recognised by certain characteristics
in each field. At Andamooka, for example,
the opal was mainly confined to a band of
silicified conglomerate some 30-40 cm thick.
Pedy the main opal
level appears at a change in the nature
of the sediments, often verified by a seam
of potch. From the base of the shaft horizontal
drives were excavated, ideally allowing
the miner to stand upright with the opal
level approximately at eye level.
The good miner would leave continuous walls
of the sandy clay rock to support the roof;
timbering is seldom necessary. A few miners
take out so much material that dangerous
conditions can be created, such as those
shown in this linked image. Under such conditions,
it is a miracle that more accidents have
At first, all work was done with pick and
shovel and lighting was by means of candles.
Opal was located sometimes by ear as much
as by sight in fields like Coober Pedy,
where the opal occurs in softer rock. The
miner could hear the scraping of his pick
on the harder, gritty opal. The opal band,
usually potch, was then followed very carefully
to avoid breakage of any valuable material
which might be encountered.
the manual hoists were replaced by electric
hoists such as the Yorke
were introduced into the drives to transport
loaded drums to automatic
hoists and dumpers for the mullock.
A further development in shifting the mullock
and 'opal dirt' to the surface has been
the use of 'blowers', a type of giant vacuum
cleaner which forced the dirt to the surface
with a high pressure blast of air through
a large, flexible tube about 0.5 metre diameter.
In recent years the shafts have been sunk
calyx drills, about a metre in diameter,
and drives excavated with large boring
There is also a tendency nowadays, especially
in Lightning Ridge, where much opal occurs
as 'nobbies' (individual stones sparsely
scattered in the sandy clay matrix) to treat
the mixed opal dirt in bulk. This began
by swirling the dirt with a jet of water
pumped up from the 'puddling' dam in what
appeared to be the barrel of an old domestic
machine (minus the agitator). The clay
and fine sand were washed away into the
bushland, the stony material being retained
in the drum.
Today, the washing machine
has graduated to the use of the large mixers
of the type used for transporting
concrete. The cleaned gravel is then
screened, sorted and examined
by hand to pick out any nobbies or other
which is washed away has been tested for
use as a pottery material. It a smooth clay,
suitable for throwing on a wheel, and, using
the appropriate techniques with regard to
its firing characteristics, has potential
as a pottery raw material. Such clay is
already being used on a small scale, and
handmade ceramics have been made and marketed
in White Cliffs and Lightning Ridge.
In some areas, especially in the boulder
opal fields of south western Queensland,
and at Mintabie, where the host rocks are
much harder, bulldozers
are used to remove the overburden. When
the opal level is reached, thin layers are
scraped away, with men following the machine
looking for opal. This system often yields
opal quickly, although significant amounts
may be lost.