Australian Gem Gallery - A Rainbow of
Gems
Opal - The National Gemstone of Australia
Australian Precious Opal - Tony
Smallwood
On the 28th July 1994, opal was declared Australia's National Gemstone.
Presently Australia supplies the world with more than 90 per cent of its
precious opal.
A little history
The Australian opal mining industry, which ultimately became a major
export earner for the national economy, began rather quietly and
hesitantly. The first discovery of opal in Australia has been attributed
to an eccentric German geologist, Professor Johannes Menge, who
discovered greenish common opal near Angaston (South Australia) some 80
km to the north of Adelaide around 1840. Precious opal was first found
in Australia on Listowel Downs Station, south of Blackall in Western
Queensland. This discovery occurred in 1868, with boulder opal mined
from this deposit being sent to England for evaluation during 1873. The
first registered opal mining lease in Australia was issued in 1871 on
the area to the south of the town of Quilpie where the historic 'Pride
of the Hills' opal mine was soon to be discovered and worked.
In 1873, precious (boulder) opal, that soon became
known internationally as 'Barcoo opal', was discovered on hills of the
Bacroo District. These opal-bearing mesa-shaped hills, which were
located to north of Thargomindah and some 160 km to the east of Listowel
Downs, where the source of the Bulloo River and its tributaries that
drained intermittently to the west. Following the discovery of further
boulder opal deposits in hills surrounding Kynuna, hundreds of
kilometres to the north, entrepreneurs, such as Herbert Bond, began,
somewhat unsuccessfully, to attempt to market Australian boulder opal to
the world. However, by the end of the 1870s, pioneer miner Joe Bridel
had discovered a new form of precious opal at Stony Creek in the Kyabra
Hills that lie north-west of Quilpie and to the south of Widorah. It was
the solid seams, 'pipes' and nodules of precious opal from this
sandstone opal that the pioneering opal marketeer Tullie Cornthwaite
Wollaston took to London in 1890 to initiate the Australian opal
industry.
Australia's first commercial opalfield was discovered
at White Cliffs in 1894, and rediscovered by two kangaroo shooters in
1889. This remote opal field is located in the arid far north-west of
News South Wales about 850 km north-west of Sydney and 200km to the
north-east of Broken Hill. From 1899 onwards, remote White Cliffs became
the world's major source of precious light to dark opal, and in
particular crystal opal. By 1899 international opal buyers were making
the long hazardous trip into outback Australia to purchase White Cliffs
opal. However, by 1904 White Cliffs was suffering as a consequence of
the 'candle box' trade, and by the outbreak of World War 1 in 1914 White
Cliffs had ceased to be a commercial source of opal.
The next commercial source of precious opal to be
discovered was Lightning Ridge ¾ the home of the famed black opal. This
opalfield, which was discovered by the Murray children when they found
traces of black opal at what is now known as 'Shallow Nobby's', was made
famous by Charlie Nettleton who walked 700 km from White Cliffs to sink
his first shaft on McDonald's Six Mile in 1903. Although this shaft was
unsuccessful, a later (1903) shaft on Shallow Nobby's yielded valuable
precious opal and commended the search for and mining of black opal
which continues today.
By 1915, the search for precious opal shifted to South
Australia. This occurred as a consequence of the discovery of opal
floaters at Stuart Range by a 14 year old boy, Willie Hutchinson. This
fortuitous discovery soon led to the establishment of the world's
largest and most productive opal field at Coober Pedy, which literally
translated from the local aboriginal dialect means "white man in a
hole". The Coober Pedy opalfield is located on the Stuart Highway
some 838 km to the north of Adelaide. Other significant discoveries of
opal soon followed in South Australia, with the remote Mintabie
opalfield, 240 km to the north-west of Coober Pedy, being discovered by
a well sinker in 1921/1922; and Andamooka, 520 km north of Adelaide on
the western shore of Lake Torrens, being discovered in 1930. From
1985-1989 Mintabie was the largest producer of precious opal in
Australia.
The search for new opalfields continues throughout
Australia, with Coocoran to the west of Lightning Ridge, Lambina in
remote northern South Australia, and Hebel across the Queensland-New
South Wales border from Lightning Ridge offering hope of an Australian
opal mining industry that will last well into the 21st century.
Where does opal occur?
Australia's economic deposits of precious opal were formed in
sedimentary environments that are closely associated with the Great
Artesian Basin or Great Australian basin. The present and historic
sources of precious opal include: black opal from Lightning Ridge
district and also the outlying fields of Coocoran, Grawin and Sheepyards.
Light and crystal opal from White Cliffs all in north-western NSW.
Light, dark and crystal opal from Andamooka, Coober Pedy and Mintabie in
South Australia, and boulder and sandstone boulder opal from scattered
Queensland boulder opal fields that stretch some 600 km from Yowah near
the Queensland-NSW border to Kynuna in the north. With the exception of
South Australia's Mintabie opalfield, each of the deposits of precious
opal are found in rocks that have a similar geological age.
How does opal form?
While small differences do exist between the geology of individual
opalfields, each of the Australian commercial opal fields share some
common features:
- A source of silica derived by protracted chemical
weathering of the sandstones of the Great Artesian Basin.
- An impermeable or semi impermeable stratum,
underlying the weathered sandstone strata of the Great Artesian
Basin, that traps downwards-percolating silica. This permeability
barrier is provided by a claystone layer in the New South Wales, a
similar strata in South Australian opal fields, and a layer of
ferruginous ironstone in the Queensland boulder opal fields.
- Passageways (joints and faults) within the
sandstone strata that allow the downwards flow of silica-rich
solutions and their entrapment at an underlying permeability
barrier. Subsequent slow evaporation of water from the trapped
silica slowly allows the formation of silica spheres which
flocculate into a mass of three dimensionally stacked structures of
silica that eventually harden into opal.
In contrast, overseas (and indeed rare Australian)
deposits of volcanic precious opal are commonly formed in volcanic rocks
such as andesite (Hungary), basalt (Honduras), and rhyolite (Mexico).
Australian deposits of volcanic opal, such as those known at Tintenbar
in NSW and Springsure in Queensland are also formed in extrusive
volcanic rocks such as basalt and pyroclastic ash flow or tuff.
It summary it is generally agreed that precious opal
forms when large amounts of terrestrial water literally wash through the
sandstone (or a similar rock) and chemically weather it. The result of
this deep chemical weathering process is the production of large
quantities of dissolved silica. This solution then percolates, under the
influence of gravity, through joints and faults in the rock strata until
it reaches an impervious level. As the silica solution comes to rest, it
spreads out along this level or fills voids or suitable sights within
the strata and begins to solidify. Eventually, further evaporation of
water takes place and opal is formed from the trapped silica solution.
What is opal?
Opal is a hydrated form of amorphous silica with the chemical formula
SiO2.nH2O, where "n" varies from 1 to 21
weight per cent. Opal occurs in two forms: precious opal with a
play-of-colour, and opal with no play-of-colour called potch or common
opal.
Precious opal is identified and characterised by its
possession of a visible play-of-colour that emanates from the opal's
oft-contrasting black, dark, or light body tone. The play-of colour of
precious opal consists of a random "pattern" of colour patches
(or grains) that show typically "spectrally pure" colours or
colours of very narrow wavelength. These colour patches often vary in
size and shape, that change hue (colour) as the location of any overhead
light source, orientation of the opal, or direction of viewing is
changed, this constitutes the pattern shown in the opal.
The cause of precious opal's play-of-colour was first
discovered and described by a group of Australian CSIRO researchers in
1965. The group of eminent Australian mineralogists included Drs P.J.
Darragh, A.J. Gaskin, B.J. Jones, J.V. Sanders, and E.R. Segnit. In
particular, the research of Sanders, Gaskin, Jones and Segnit revealed
that the play-of-colour in precious opal was caused by three dimensional
close packing of its constituent transparent spheres of non-crystalline
silica. These spheres have refractive indices that usually ranges from
1.435 to 1.455. As the diameter of these spheres approaches that of the
wavelength of visible light, the regular three-dimensional stacking of
similar sized spheres produces an array of tiny voids or holes between
contacting spheres. These sphere-void interfaces act as light scattering
points that form a diffraction grating at which incident white light
will be bent (diffracted) into its constituent red, orange, yellow,
green, blue, indigo, and violet wavelengths. So, when the diameter of
the silica spheres, and hence the spacing of the voids between adjacent
spheres, is of the same order as the wavelength of visible light, the
spectral colours of precious opal's play-of-colour will be generated.
Simply put, as the colour or wavelength observed in
the play-of-colour of precious opal is directly related to the size of
its constituent spheres: blue wavelengths will be diffracted by the
smaller sized spheres of ~140 nm (nanometres) diameter. Red wavelengths
will be diffracted by larger sized spheres of ~240nm and as large as 300
nm diameter. Also, as a precious opal is rotated through different
angles of viewing, if the spheres of silica are large enough to diffract
red wavelengths of light then … as the opal is rotated the full
rainbow of spectral colours will be progressively observed, starting
with red and progressing through orange, yellow, green, blue, indigo,
and violet.
Gemmological properties of precious opal
Australian precious opal from the sedimentary environment is a singly
refractive non-crystalline variety of silica that has a water content
usually of 3 -10 per cent by weight. It has a hardness of 5 to 6½ on
Mohs' comparative scale of scratch hardness; is not particularly tough;
and when fractured will produce a conchoidal (shell-like) fracture
surface. Australian precious opals are divided into opal (type 1),
boulder opal (type 2) or matrix opal (type 3), and have body tones
(relative lightness or darkness) that range from black through dark to
light. The diaphaneity varies from completely opaque to completely
transparent. Transparent opal that displays a play-of-colour is commonly
described as crystal opal. Polished surfaces of precious opal typically
display a resinous to sub-vitreous lustre. However, exceptional
specimens of Australian precious opal may display a vitreous lustre.
These Australian precious opals usually have specific
gravities a range from 2.10 to 2.20 (mean 2.15) and are singly
refracting, yielding a refractive index range from 1.44 to 1.47 (mean
1.45). When exposed to long-wave ultraviolet of 365 nm wavelength (LWUV),
most Australian sedimentary precious opals emit a bluish white
fluorescence of moderate to strong intensity. The intensity of this
fluorescence is strongest in white and light opals and weakest in black
opal. In contrast, the opal's response to short-wave ultraviolet light (SWUV)
is much more subdued. Importantly, following irradiation with
ultraviolet light, Australian sedimentary precious opal displays a
yellowish green phosphorescence of moderate to strong intensity that may
last up to twelve (12) seconds after the ultraviolet source is
extinguished. Some Queensland boulder opals show may show variable
luminescence.
Inclusions commonly found including Australian
sedimentary precious opals include: potch (non precious opal) and potch
lines, sand, gypsum, ironstone (in boulder opals), internal fractures
(crazing), spider web-like superficial 'webbing' tend to follow the
patterns of colour in the opal, water containing fluid inclusions, and
finely dispersed minerals of various type.
The imposters
Due to intrinsic value of Australian precious opal, these are often
imitated by a range of man-made materials that include treated natural
opal, composite natural opals (doublet opals, triplet opals, mosaic and
chip opals), so-called synthetic opals, and opal imitations.
Treated opals are opals that either have had the
visibility of their play-of colour enhanced by impregnation of their
matrix with a black pigment (e.g. carbonised Andamooka type matrix
opal), or have had their body colour or porosity enhanced by plastic
impregnation (e.g. black polymer impregnated crazed opal).
Doublet opals are a two part composite of natural opal
cemented to a base material.
Triplet opals are a three part composite, that
consists of a slice or laminate of natural opal cemented between a
transparent top layer and a darkly coloured base material.
Mosaic and chip opals are composites in which small
pieces of opal are cemented to a base material.
Synthetic opals are laboratory-manufactured opals that
have essentially a similar chemical composition and structure as natural
opal; but are bound together by polymers or other inorganic materials
not found in natural opal. Present day manufacturers of synthetic opals
include Pierre Gilson (the original manufacturer), Inamori (a Japanese
manufacturer), an unspecified Russian and Chinese manufacturers.
Fortunately, synthetic opals can readily be identified by characteristic
irregular or crenulated shape of the grains or colour patches that form
this man-made opal's pattern.
Imitation opals are opal "look-alikes" that
are manufactures from any material that can simulate the play-of-colour
of precious opal. These man-made materials may include metal foil
included glass, iridescent glasses of complex composition such as Slocum
stone, three dimensionally stacked polystyrene sphere imitations of
opal, and thin film holograms that generate a pattern of colours that
resembles precious opal's play-of-colour.
The value of an opal
Determining the quality and assigning a value to a precious is a very
skilled process that is best left to those who handle opal on a
day-to-day basic. Nevertheless, and in very general terms, the quality
of a precious opal may be determined and quantified by considering the
following factors of an opal when viewing the gem face up:
-
The gemmological identity of the material.
Is the opal precious opal or common opal?
Is the opal natural or synthetic?
Is the opal treated?
Is the opal solid or composite?
-
Type of natural opal.
Is the opal all opal? - natural opal
Is the opal, opal and rock? - natural boulder opal.
Is the opal, opal in rock? - natural matrix opal.
-
Body tone of the opal
Is the body tone Black or Dark?
Is the body tone Dark or Light?
-
Diaphaneity of the opal
Is the opal translucent or opaque?
Assessment of the observable play-of-colour.
This involves the determination of six factors that are shown by the
play-of-colour of the opal and is the most difficult factor to judge for
any opal:
- Its brightness … How bright is the overall
play-of-colour?
- Its spectral range … What range of colours is
visible in the play-of-colour ?
- Its saturation … How pure and vivid are the
colours forming the play-of-colour
- Its pattern … What is the size, shape, regularity
and rarity of the play-of-colour?
- Its consistency… Is the play-of-colour, pattern,
brightness consistent or variable over the whole face of the opal?
- Its directionality … Is the play-of colour
visible from all directions as the opal is rotated?
Assessment of clarity
All opals must be visually assessed for face-up clarity. Any eye visible
inclusion, such as patches or lines of potch, 'webbing', the presence of
'sand spots' and/or crystals of gypsum near the surface or the opal, or
the presence of ironstone (in the face of boulder opal), will effect the
value of an opal. Importantly, the presence of crazing (surface-reaching
cracks and fractures) in precious opal renders it commercially
valueless.
Quality of cutting
Precious opals are usually cut en cabochon as ovals (seldom) rounds, and
in other 'free form' shapes. Cut and proportion factors to be considered
when assessing opal include:
- Shape … oval, round or free shape.
- Dome … high, medium, or low dome, or flat faced.
- Proportion of play-of-colour … What is the
proportion of the play-of-colour to the proportion of potch or
ironstone?
- Window … Is there a 'hole' in the opal that
allows light to enter through the back of the stone and so dilute
its play-of-colour?
Carat weight
As the weight of the opal also affect its value, prices per carat will
increase up to about 10 carat, after which as larger sizes become more
difficult to use common items of jewellery (eg. rings and earrings),
value (per carat) may decrease.
Field of origin
While the field of origin of an opal should not influence its value,
economic reality suggests that this does occur. However, due to the
difficulty even experts have in determining the precise and unequivocal
origin of some precious opals, it is suggested that the origin of an
opal is best described by prefixing its 'type' (eg. Lightning Ridge type
black opal) to the opal's detailed description.
References:
Barnes LC, Townsend IJ, Robertson RS & Scott DC, (1992) Opal: South
Australia's gemstone. Handbook No 5. Geological Survey of South
Australia: Adelaide.
Cram L, (1994) Beautiful opals: Australia's National Gem. Robert Brown
& Associates: Coorparoo.
Cram L, (1998) A journey with colour. Cram: Lightning Ridge.
Smallwood AG, (1997) A new era for opal nomenclature. Australian
Gemmologist. 19, 486-496.
Smallwood AG, (1998) Opal module, course notes. GAA (NSW Division):
Sydney.
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