Fossilised opal shells are rarely seen by any living human being. They occurred when in the Great Artesian Basin the inland seas began to recede. The bivalve mussels and other shellfish were left behind, died and were buried in the sand and mud as the sea receded. Of course, they were sealed as the mud solidified and the calcites, or calcium carbonate of the shells, began to disintegrate along with the internal remains of the mussel or shellfish. This would often leave holes or gaps in the solidified sediment.

Occasionally, opal would begin to percolate down into these gaps, a rare occurrence. The hydrated silica, common opal known as Potch, would fill these gaps in the solidified mud of the sediment. The calcite would disintegrate and occasionally be replaced by common opals of white, grey, brown, transparent crystal, and very rarely black opal.

In very rare occurrences, the hydrated silica would form a diffraction grating, and when brought to the light would show red, orange, yellow, green, blue, indigo or violet, or combinations of those colours depending on the formation of the diffraction grating at the molecular level. In this instance, one of the rarest fossils on earth would produce a colourful crystal opal or, even more rarely, a black opal with colour.

The opal itself would have variable optical density. If the colour, the diffraction grating producing the colour, was right throughout the fossil, skin-to-skin colour would appear. This would produce a shell that had colour all the way through on both sides. When these skin-to-skin coloured opals are found, they are extremely valuable and make exceptional pieces of jewellery for collectors of rare and exciting pieces of opal. These pieces are right up there in rarity and desirability with the rarest black and boulder opal.

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