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Australian Bolder Opal

Queensland:
produces the bright and beautiful Boulder
Opal, which is mined around the Quilpie area.
Opal forms naturally on ironstone or ``Boulder"
which is cut to form part of the gemstone and is
viewed as a solid opal.

The bolder opal used by most of our suppliers
comes from the Queensland areas. Below is an
article we found that tells about. The type of
patterns found in opal, bolder opal varieties,
formation and history.

Patterns:
Another unique feature of precious opal is the pattern of colours associated with the play of colours. A whole range of patterns differentiate the various types. Some of the more common are listed here.
Harlequin:
opal has a mosaic or chequered pattern of coloured patches. It is the rarest and the most sought after.
Fire opal:
shows flashes of predominantly red, orange, and yellow within transparent to translucent stones of pale yellow to red body colour.
Pin fire:
is composed of closely spaced specks of brilliant colour.
Flame opal:
the colour may be seen as red bands or streaks.
Flash opal:
it appears as sudden brilliant flashes.
Girasol:
the play of colours appears as a floating internal light.

BOULDER VARIETIES:
Like other precious opal, there are many varieties of boulder opal defined on body colour,
play of colour, and patterns. A specific nomenclature for the variations has been preposed
by the Australian Gemmological Association to standardise the names for miners, gemstone buyers and all associated with the gemstone industry.
Black boulder opal:
This rare, very valuable Queensland opal rivals Lightning Ridge black opal in both the
uniform darkness of its black body colour, and the contrasting range of complete spectral
hues visible in its play of colours.
Crystal boulder opal:
These transparent boulder opals owe their dark body colour to the dark brown colour of
the ferruginous sandstone or ironstone backing that may be observed through the polished surface of the opal.
Light boulder opal:
Light coloured translucent to opaque opal with a ferruginous sandstone or ironstone backing. Boulder matrix opal:
An anastomosing network of precious opal veins within ferruginous sandstone or ironstone. Yowah Nuts:
Small rounded opaliferous ironstone concretions that may host either solid opal or opal
matrix. The nuts are small ironstone boulders which are spherical to ellipsoidal in shape
and up to five centimetres across. They may have hollow centres or be filled with powdery
clay or a kernel of opal. Opal is found mainly between concentric layers on the underside of the nuts or as a network of thin veins through the ironstone concretion. The latter is
frequently termed opal matrix. The best development of this type of opal is at Yowah
where the concretions form distinct nut bands which are commonly associated with layers
of mudstone clasts or clay pellets.
Fun stones:
Attractive and comparatively inexpensive fee-form boulder opals of lower quality
Boulder splits:
Amatched pair of boulder opals formed by splitting rough boulder opal along a flat vein.
Seam and vein opal:
Very rare boulder opals cut from the thin irregular veins of opal that occasionally occur in
the ferruginous seam (casing), or sandstone above the seam, or in the claystone below the seam. The thin ironstone casings are up to five centimetres thick at the contact between sandstone and underlying fine-grained sediment. In some places, the upper surface of the seam has rounded, botryoidal protrusions (nobbies) up to several centimetres across, and
both seam and nobbies commonly contain thin, horizontal veins and random flecks of
brilliantly coloured precious opal. Immediately above the seam, opal dirt is commonly found.
Sandstone Opal:
Dark brown, ferruginous sandstone impregnated with pin points of precious opal. The sandstone opal is formed by replacement of the matrix of the sandstone and is found impregnating the ferruginous sandstone 20 t 30 centimetres above the seam.


REGIONAL GEOLOGY:
Distribution of opal deposits in Queensland is extremely wide and erratic in contrast to the relatively confined occurrences in New South Wales and South Australia. Queensland's productive fields lie within a belt of Cretaceous sedimentary rocks - known as the Winton Formation - which extends from the New South Wales border at Hunderford in a
northwesterly direction to Kynuna, a distance of about 1000 kilometres. The opal belt stretches to the west of the railheads at Cunnamulla, Quilpie, Longreach and winton, and encompasses the smaller centres of Eulo, Eromanga, Windorah and Jundah. Mining
activity is concentrated mainly in the opal fields within the belt, although some scattered operations are outside these areas previously described.

Staff of the Bureau of Mineral Resources and the University of New South Wales have studied the geology of western Queensland in detail and have arrived at an explanation of
the sequence of geological events which were crucial to the formation of opal. During Cretaceous time, the sedimentary rocks of the Winton Formation were deposited in streams and lakes. A period of deep weathering affected these rocks during latest Cretaceous to
early Eocene time (50-65 million years ago) causing the formation of the tri-layered Morney Profile to depths of up to 90 metres.

In this profile an upper siliceous zone overlies a varicoloured zone, which in turn overlies a basal ferruginous zone. Iron oxides leached down from the overlying rocks wee chemically precipitated as concretionary ironstone in the basal ferruginous zone. Development of drainage patterns saw sedimentation along river systems with fragmentation and minor
erosion of the Morney Profile along interfluves. A second weathering event in the late Oligocene (approximately 25 million years ago) formed the morphologically distinct Canaway Profile, consisting of an indurated crust and mottled zone grading down into varying thicknesses of the residual older profile and in places merging laterally into it. The depth
of the indurated kaolinitic breccia which formed the crust extended down some 10 to 14
metes, with the full profile extending to about 40 metres depth.

Silica was released during this second stage of weathering and migrated downwards as an aqueous sol. Where erosion had removed sufficient of the older profile to bring he basal ferruginous zone to within 40 metres of the surface, this silica was able to collect and precipitate as opal within voids in the ironstone host rocks. The Canaway Profile apparently formed along interfluves and may have developed at the same time with surface silcrete
across adjacent plains mantled by quartzose clastics. Opal deposition and silcrete formation were probably contemporaneous.

The crust and mottled zone of the Canaway cut across the former tri-layered arrangement of the parent (Morney) profile. The convergence of the basal ferruginous zone (of the Morney) with the indurated crust (of the Canaway) appears to be an important factor in opal formation. Where the crust and basal ferruginous zones are in close proximity, the host ironstone bodies were favourably located within a fluctuating groundwater table which permitted deposition
and dehydration of siliceous material. Geological processes (folding, faulting, erosion) to the present day have either completely removed these weathered profiles or left remnants of
them as flat-topped landforms (mesas) or low rises.

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