Behind the Opal Essence
A guide to Opal quality
A guide to Opal quality
The Boulder opal found in Queensland forms in a slightly different method to other types of opal, forming inside an ironstone concretion. The concretion was formed due to ionisation, from sedimentary deposition. By definition, they are ionised concretions of varying hardness with an approximate opal composition of SiO2at 28%, Fe2O3 + AL203 at 68% and H2O at 1% composition.
Fe2O3 + Al2O3
Iron Oxide + Aluminium Oxide
The opal forms in generally elongated or ellipsoidal ironstone concretions or boulders, from a few centimetres, to up to 3 m across. The boulders may be confined to one or more layers or randomly distributed through the weathered sandstone. Their composition ranges from sandstone types (a rim or crust of ferruginised sandstone surrounding a sandstone core) or ironstone types (composed almost entirely of iron oxides). The opal occurs as a filling or lining between the concentric layers or in radial or random cracks in the ironstone, or as a kernel in smaller concretions or nuts. (as found at Yowah and Koroit fields, the famous 'Yowah-nuts'). Example: Aurum’s ‘Viridian’ Butterfly Brooch.
Matrix opal is where the opal occurs as a network of veins or infilling of voids or between grains of the host rock (ferruginous sandstone or ironstone). Rare seam or band opal is also found and is typically encased in ironstone.
Pipe opal occurs in pipe-like structures which may be up to several centimetres in diameter within the sandstone and these structures may be hollow or opal-filled. Wood opal is occasionally found replacing woody tissue material.
As opposed to other sedimentary precious opal, boulder opal is attached to the ironstone, and stones are usually cut with the natural ironstone backing intact. Solid opals may be cut from the ironstone material where the opal is of sufficient thickness.
Boulder opals are fashioned to standard shapes and sizes but are also cut in freeform shapes to highlight their individual beauty and to avoid wastage. Example: Aurum’s ‘Chartreuse’, ‘Azure’, and ‘Periwinkle’ Pendants. Magnificent picture stones are also cut but these are mainly of interest to collectors rather than for jewellery use.
Boulder opal also has a dark body tone (although there are some occurrences of white boulder opal), which is as dark as black opal. Because boulder opal forms in thin veins in ironstone boulders, the host ironstone is usually left on the back of the stone. This is why, if you look at the back of a boulder opal, there is a layer of brown rock attached to the back. For this reason, boulder opal is sometimes referred to as a 'natural doublet', as this layer of dark stone on the back gives the opal its dark body tone.