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Opal is a hydrated amorphous form of silica; its water content may
range from 3 to 21% by weight, but is usually between 6 and 10%. Because
of its amorphous character, it is classed as a mineraloid, unlike the
other crystalline forms of silica, which are classed as minerals. It is
deposited at a relatively low temperature and may occur in the fissures
of almost any kind of rock, being most commonly found with limonite,
sandstone, rhyolite, marl, and basalt.
Opal is the national gemstone of Australia. Australian opal has often
been cited as accounting for 95-97% of the world’s supply of precious
opal, with the state of South Australia accounting for 80% of the
world’s supply. Recent data suggests that the world supply of precious
opal may have changed. In 2012, Ethiopian opal production was estimated
to be 14,000 kg (31,000 lb) by the United States Geological Survey. USGS
data from the same period (2012), reveals that Australian opal
production to be $41 million. Because of the units of measurement, it is
not possible to directly compare Australian and Ethiopian opal
production, but these data and others suggest that the traditional
percentages given for Australian opal production may be overstated. Yet,
the validity of data in the USGS report appears to conflict with that
of Laurs and others and Mesfin, who estimated the 2012 Ethiopian opal
output (from Wegal Tena) to be only 750 kg (1,650 lb).
The internal structure of precious opal makes it diffract light;
depending on the conditions in which it formed, it can take on many
colors. Precious opal ranges from clear through white, gray, red,
orange, yellow, green, blue, magenta, rose, pink, slate, olive, brown,
and black. Of these hues, the reds against black are the most rare,
whereas white and greens are the most common. It varies in optical
density from opaque to semitransparent.
Common opal, called “potch” by miners, does not show the display of color exhibited in precious opal.
11.0 x 7.0 x 2.5 mm
Also known as ‘milky opal’, white opal features light white body
tones, and is mined in South Australia. White opal is more common and
because of its body tone, generally does not show the colour as well as
black opal. Nevertheless, white opals can still be absolutely
magnificent in colour if a good quality stone is found.
7.75 ct. Lightning Ridge Crystal Opal Credit: Mardon Jewelers
Crystal opal is any of the above kind of opal which has a transparent
or semi-transparent body tone – i.e. you can see through the stone.
Crystal opal can have a dark or light body tone, leading to the terms
“black crystal opal” and “white crystal opal”.
Is a term sometimes mistakenly and improperly used to refer to fire
opals, as well as a type of transparent to semitransparent type milky
quartz from Madagascar which displays an asterism, or star effect, when
cut properly. However, the true girasol opal is a type of hyalite opal
that exhibits a bluish glow or sheen that follows the light source
around. It is not a play of color as seen in precious opal, but rather
an effect from microscopic inclusions. It is also sometimes referred to
as water opal, too, when it is from Mexico. The two most notable
locations of this type of opal are Oregon and Mexico.
Peruvian Opal (also called Blue Opal)
Is a semiopaque to opaque blue-green stone found in Peru, which is
often cut to include the matrix in the more opaque stones. It does not
display pleochroism. Blue opal also comes from Oregon in the Owhyee
region, as well as from Nevada around Virgin Valley.
Fire of Australia
The finest uncut opal in existence, the Fire of Australia, has joined
the South Australian Museum’s collection through the vision of a
private donor and funding from the Federal Government’s National
Cultural Heritage Account.
Valued at nearly $900,000 Australian dollars and weighing at 998
grams, the Fire of Australia is the world’s finest piece of opal of its
kind on public display.
The Director of the South Australian Museum, Brian Oldman said the rarity of this piece of opal cannot be underestimated.
“Opal of this quality can only be created under certain climate conditions,” Mr Oldman said.
“90% of the world’s most precious opals are found in South Australia.
“When our state’s inland sea evaporated millions of years ago it
provided a unique silica-rich environment for the creation of precious
opal. It is these exceptional conditions that created the Fire of
Australia.”
Still in the rough condition in which it was found, two faces of the
Fire of Australia have been polished to reveal the gem’s exceptional
quality, with its transitioning colour from green to yellow to red
depending on the angle from which it is viewed.
Minister for the Arts the Hon Senator Mitch Fifield today announced
$455,000 in federal funding for the Museum to secure the significant
piece.
The Turnbull Government understands the importance of preserving and
displaying Australia’s unique artefacts locally for current and future
generations.
This funding helps Australia’s cultural institutions, such as the
South Australian Museum, acquire significant objects for public display.
Walter Bartram’s son Alan said that the Fire of Australia was mined
in 1946 by Walter Bartram at the Eight Mile field in Coober Pedy, South
Australia and has been in his family for over 60 years.
“After loaning the Fire of Australia to the South Australian Museum
for its Opals exhibition, we made the decision to place this family
heirloom in safe hands.
“We’ve been long term supporters of the South Australian Museum and
it seems fitting that it should be passed onto the people of South
Australia to enjoy,” Mr. Bartram said.
Opals was the most visited paid for exhibition in the Museum’s
history, resulting in donations of precious opals of more than $3
million, which includes the Fire of Australia.
The Fire of Australia opal will be on display in the South Australian Museum’s front foyer until February 28 2017.
Discovery
The gem was first discovered in 1946 by miner Walter Bartram at the
Eight Mile opal field in Coober Pedy — a small desert town in South
Australia famous for its opals.
(South Australia, which encompasses a vast arid area in the south and
middle of Australia, produces more than 90% of the world’s precious
opal, according to Oldman.)
Oldman said it would have been part of a larger seam of opal that ran underground, and would have been extracted in pieces.
Representative Image
Scientists are using the new Geoscience Atom Probe Facility at Curtin
University to study mineral deposits containing locked resources of
gold in refractory ores.
Curtin WA School of Mines Research Associate in Applied Geology Dr
Denis Fougerouse and fellow researchers have found metallic gold
nanoparticles only a few nanometres in diameter within the mineral
arsenopyrite – a common mineral found in Australian mines.
Dr Fougerouse said the study was believed to be one of the first of
its kind, and the discovery challenges the understanding of nanoparticle
formation and allowed the team to establish the main controls on gold
incorporation in sulphides.
“The application of atom probe microscopy in geosciences is
relatively new. The technique is based on field-evaporation of atoms
from tiny, needle-shaped specimens to provide three dimensional
sub-nanometre scale information of the position and type of individual
atoms in the specimen in the mineral,” Dr Fougerouse said.
“Typically, the amount of material analysed is really, really small –
a single grain of salt is over a billion times larger than a typical
analysis.”
Dr Fougerouse explained large resources of these nanoparticles are
‘locked’ in gold-bearing arsenopyrite, an iron arsenic sulphide, which
can be found in mines across the world.
“Arsenopyrite is a very common mineral found in Australian and other
mines, and although not every arsenopyrite contains gold, it is common
to find gold locked inside this mineral,” he said.
“Our results show that gold can be hosted either as nanoparticles or
as individual atoms in different parts of the crystal structure, and the
different types of gold yield important information about the controls
on gold deposition as the ore body forms.”
Dr Fougerouse explained this study demonstrated the capability of atom probe microscopy in geosciences.
“Our research shows the Geoscience Atom Probe has potential to
characterise gold deposition processes at the atomic level. In turn this
could help unlock hidden gold resources in known deposits, and will
enhance gold recovery,” Dr Fougerouse said.
“Nanogeoscience is a new, but rapidly growing research field. Through
this research and use of the Geoscience Atom Probe, we can show that
tiny observations can yield big results that have potential economic
importance.”
