Page 14 - 2018 Coober Pedy Visitor Guide

Opal Information

“For in them you shall see the living fire of ruby,
the glorious purple of the amethyst,
the sea green of the emerald,
all glittering together in an incredible mixture of light”
Written by Roman Pliny in the 1st Century AD
by Kezia Manning
Australia has 95% of the world’s supply of Today truck-mounted blowers which operate
commercial opal and the largest like vacuum cleaners are more commonly
percentage still comes from the 70 opal used.
fields around Coober Pedy.
Opal is a true precious stone composed
of amorphous silica and water. 85% of all
opal found is called potch (opal without
fiery colours) which has little value. It is
the other 10% of vividly coloured opal that
qualifies as the precious opal admired
throughout the world.
Mining Opal
Miners, with a Mining Permit, can peg a by Kezia Manning
claim either 50m x 50m or 50m x 100m to
mine for opal. Opal Processing and Classing
The earlier form of mining was by sinking Once the rough or raw opal has been
or digging a shaft then tunnelling along mined it is placed in a small water-filled
the level with a pick and shovel. A hand- tumbler or a cement mixer. The agitation
pick or screwdriver was used when traces cleans about 95% of the dirt (sandstone)
of opal were found. from the rough opal and whatever dirt
Since the 1970’s, there has been a rapid remains is then snipped off.
increase in the use of mining machines. The opal is sorted or classed in different
These days, most if not all prospecting grades by size and of course colour and
shafts are made by using a Calweld-type quality. Colour in precious opal is caused
drill which is used to excavate holes about by the regular array of silica spheres
one metre in diameter using an auger diffracting white light and breaking it up
bucket. The drills can dig to a maximum into the various colours of the spectrum.
depth of about 28m to 30m. There are a number of things to assess
Today’s opal fields are subsequently pitted when classing opal colour. These include
with thousands of abandoned Calweld the quality, brilliance, clarity, variety and
shafts. pattern.
Tunnelling machines with revolving The main factors in determining the price
cutting heads and small underground paid for opal are as follows.
front-end loaders called boggers are used
while bulldozers are employed to remove
overburden and expose the level where it
is shallow.
Spotters follow behind watching for opal
and the seam is then worked over by
hand-pick.
Waste material (or mullock) from the
shafts and drives was originally lifted to
the surface by hand windlass, then by
power winches (Yorke hoists) or automatic
bucket tippers. by Ishka Maat
Coober Pedy
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