Gems from a desert ocean - Origin of Opal

After clusters of shellfish, large marine animals, or waterlogged tree trunks were buried under sea floor mud, the fossils were encased in a cement-like material. This casing preserved the shells, bones and wood as the mud was compressed and turned to shale-rock.

Millions of years later after the Eromanga Sea had vanished, there were periods of wet climate when lakes and forest covered the inland of Australia. Acidic ground water from decaying vegetation percolated down into the old fossil bearing rock. The limey concretions and fossil shells and bones were dissolved away and the opal filled the spaces left by shells, bones or even fractures in the rock.

All precious opal in South Australia occurs in rocks affected by weathering during the Tertiary Period, some 15 to 30 million years ago. Rock that lay below the weathering zone was unaffected by the opal forming process.

Photo by Jo Bain

Opal shell blocks

This uniquely preserved shell bed was excavated from an opal mine in Coober Pedy. The sample of an Eromanga sea floor community consists of densely packed layers of opalised cockles and mussels, with smaller numbers of fan shells (Maccoyella), brooch shells (Trigonids), and rare moon snails (Euspira).

Cretaceous Shellfish:

• Filter feeders

• Carnivores

The opal shell-block is a snapshot of life on the sea floor 120 million years ago frozen in time. It was carefully 'unpacked' by Ana Glavinic who exposed each shell with fine drill bits so that they can be identified. Photos by Jo Bain.

What is opal?

Opal is a form of silica, chemically similar to quartz, but containing water within the mineral structure. Precious opal consists of small silica spheres, of uniform size and arranged in a regular pattern.The colour in precious opal is caused by the regular array of silica spheres diffracting white light and breaking it up into the colours of the spectrum.

The play of colour in opal depends on the angle of incidence of the light and can change or disappear when the gem is rotated. The size and spacing of the spheres also controls the colour range of opal.

In opals showing reds flashes, the spheres are larger than those showing only violet or green flashes.In potch opal and common opal the silica spheres may be absent or too small or irregularly arranged to produce colour.

Many of the fossils in this exhibit are on display as a result of generous donations and loans by members of the public.

Scanning electron micrograph of the silica spheres in opal.