Rough Boulder Opal***1000ct.***/Nr.203

OPAL CLASSIFICATION

Terms such as boulder opal, sandstone opal, matrix opal and pipe opal were somewhat ambiguous, there being considerable variation in the meaning of the different terms.   Furthermore, the terminology of the opal miner is rather vague, with some words meaning different things to different people.  The following definitions may allow some uniformity.

There are many types and varieties of precious opal, but western Queensland is renowned for boulder opal.

Boulder opal describes opal found within ironstone concretions of varying shape and size.  The boulder shape is generally elongated or ellipsoidal with the long axis orientated horizontally, and the size ranges up to three metres in length and breadth and one metre in thickness.  The direction of elongation usually parallels the bedding in the enclosing sandstone.  The boulders may be confined to one or more zones known as boulder levels, or may be erratically distributed through the sandstone.  Their composition ranges from sandstone types which consist of a rim or crust of ferruginised sandstone surrounding a sandstone core, and ironstone types which are composed almost totally of iron oxides.  Commonly, these concretions have a concentrically banded structure and precious opal may fill concentric, radial, or random cracks, particularly on the underside of the concretion.

Only a small proportion of boulders contain precious opal of any economic value. Boulder opal is widely distributed throughout western Queensland and is the State's most significant source of precious opal.

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/Koroit Boulder Opal - small rounded opaliferous ironstone concretions that may host either solid opal ,opal matrix or just a bretty structure/Buttons. A attractive and comparatively inexpensive fee-form boulder opals of lower quality!

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/Koroit where the concretions form distinct nut bands which are commonly associated with layers of mudstone clasts or clay pellets.

Boulder splits - a matched 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.

Pipe Opal - crystal opal cut from solid opal recovered from unique pipe-like structures. Sometimes sandstone is retained on the backs.  

The pipes may be up to several centimetres in diameter and may be hollow or opal filled.   They may swell and thin or coalesce with others to form knotted masses.  Their formation is possibly joint controlled, or they may represent zones of higher permeability within the sandstone which acted as channel-ways for descending waters.

Wood Opal - refers to opal which forms in tube-like shapes by the replacement of woody tissue, particularly in the opal dirt or in the mudstone underlying a seam.