Opal Fossil sea Shells

I look after more than 30,000 mineral specimens, many of them exquisitely beautiful. This little sea shell is one of my favourites. It is Ampullospiro sp., a fossil gastropod, just 2.5cm across, and it is composed of precious opal which flashes beautiful colours as it is turned in the light. It comes from the opal mining fields of Queensland, Australia, and was purchased by the Museum sometime before 1896.

This little gastropod lived in the sea or a river estuary in early Cretaceous times, around 110 million years ago. After it died, it was buried by sediments which turned into rock. Heavy weathering left voids in the rock where fossil shells, wood, and reptile bones had been, and these filled up with opal. If you look at opal using a scanning electron microscope, you can see it is made up of tiny spherules of silica stacked up like a heap of ping-pong balls. Most opal looks very dull, but in rare precious opal, the spherules are just the right size to diffract the light, breaking it into its spectral colours.

This delicate little shell has both age and beauty.

Monica Price, Mineralogical Collections