PLAY OF COLOUR
Areas within an opal that produce a play of colour [precious opal] are made up of microscopic spheres of silica arranged in an orderly network. As white light passes through the array of spheres it is refracted into the beautiful colours of the light spectrum. The size of the spheres and the regular or irregular geometric packing determine the colour and quality of the refracted light.
So the more perfect the array of spheres the brighter the colour and pattern of the stone.
TYPES OF OPAL
Chemical formula Si02.n[H2O]
BLACK OPAL is the most valuable opal and comes mainly from Lightning Ridge. High quality stones are rare. Black opal has an intense colour bar sitting on a deep dark to black base, which is formed naturally in situ beneath the colour bar. The dark colour of the base enhances the brightness and intensity of the colour bar above.
BOULDER OPAL, mined in Queensland is precious opal formed in siliceous cracks and voids of the host ironstone material, in the form of surface and subsurface ironstone boulders . They are cut and shaped with their natural ironstone backs and are categorised as solid opal. Their often irregular shapes add to their aesthetic appeal and allow them to be used in a variety of jewellery designs.
CRYSTAL OPAL is simply the colour bar of opal without a back or base. Light can pass freely through the body of the stone, and sometimes has a darker body colour, this we call a dark or black crystal opal.
LIGHT OPAL\WHITE OPAL is found on all opal fields in Australia, but mainly in Coober Pedy SA, and White Cliffs NSW. It has a white or light base or body tone.
SOLID OPALS are 100% natural, and must be cut and\or polished from the natural stone without being treated or added to in any way. Solid opals are the most valuable type of opal. They can be cut as cabochon, free shape or free form carvings.
DOUBLET OPALS are made by gluing slices of precious opal to a piece of dark backing material, with blackened epoxy resin. The dark backing enhances the gem colours of the opal slice.
TRIPLET OPALS are made by adding a clear protective cap of quartz, or crystal glass to a doublet. The colour layer is usually thinner than on a doublet.
OPAL CLASSIFICATION
Black opal N1 to N4
Dark opal N5 to N6
Light opal N7 to N9
BODY TONE background colour or body colour all refer to the relative darkness or lightness of the opal, while ignoring its play of colour. This is assessed on a scale of body tone from N1 to N9.
POTCH is a term used to describe common opal with no play of colour; it can be black, grey, white or even light blue or magpie [black and white]
INLAY OPAL is a technique that uses a solid slice of precious opal, much like a doublet slice, but rather than being glued onto a dark base, it is fitted straight into the silver or gold setting creating a smooth clean line, between metal and opal, the result is a very stylish and neat item of jewellery at a reasonable price.
A GUIDE TO OPAL
Captured by nature all the colours of the spectrum are returned in an infinite variety of shades, patterns and brilliance from the depths of this most rare gemstone.
Having laid dormant in the strata of the earth for millions of years, their beauty hidden from view, a miners pick chips the edge of a black opal nobbie (nodule) and for the first time bright light fills the micro structure of what miners call the colour bar, exposing the wondrous hues and patterns unique to that particular stone. Unique, for no two stones are ever exactly the same. Sometimes stones from one area may be very similar, but this is a rare occurrence.
Opal creates its colour in the same way as the rainbow. White light passing through the billions of closely packed spherical silica particles, arranged in a regular array, is then refracted into different wave lengths allowing us to see and enjoy the beautiful colours of the spectrum.
These silica spears are so small it would take 20,000 to cross smallest dot from a ball point pen.
Opal Occurrence
A large section of eastern Australia was once covered by a shallow inland sea that stretched all the way from lightning ridge (N.S.W.) to Cooper Pedy (S.A.) to Winton (Q.L.D.) The precious Opal bearing clay levels that Lightning Ridge miners now work were laid down over millennia as the beds of the various lakes, estuaries and rivers of that now “lost inland sea” region. The ancient land after which “Lost Sea Opals” was named.
Opal occurs when minute particles of silica, slowly over millions of years, fill small fissures and voids in the cretaceous clay strata. Geologists believe the silica particles were eroded and transported by surface water moving through the upper levels of gravel before being halted by the impermeable clay levels below, intern depositing the silica within the cracks and voids of the clay beds. Further theories suggest very hot siliceous rich fluids pushed up under great pressure from deep volcanic activity, have also brought suitable material to the site. This combination of minerals, heat, pressure and time allowed the silica to deposit into perfectly aligned arrays of regular sized molecules and form the layers of the colour bar that make up precious Opal.
All this adds up to the amazing natural formation of gem quality opal, but Lightning Ridge still has one more aspect to add to the equation, in the form of a deep black layer of opal attached to the colour bar. Formed in situ this black opal base enhances and enriches the colour bar above giving a depth and vibrancy of colour that no other gemstone can truly match. All opal is amazing and beautiful but Lightning Ridge went one better with Australia’s national gemstone the famous Black Opal!