Opalised fossils are one of natures greatest treasures. Opal can hide many surprises. Because opal was generally formed at the bottom of the sea or swamp, it has replaced many interesting things. Sea shells, fresh water shells, sea creatures, bones and plants have all been found transformed into beautiful opal. One of my earlier posts, Opalised Shells shows some examples of shells which have been opalised.
In most cases the opalised shells or bones have been formed when the actual shell or bone has dissolved away at the bottom of the sea or swamp and the remaining cavity has been filled with the silica gel in exactly the right mix and conditions. Examples of these shells and other fossils can be seen on the Australian Opal Centre website.
I am sorry to say Andre has asked me to remove all of his photos from my site.