Over the many years of mining at Lightning Ridge I have found several of these Yabby buttons (including the ones here) But I have found only a few with color. The ones with color can be worth high dollars.These fossils are over 100 million years old.

Below is an article from the Australian Opal Center about yabby buttons:
Freshwater crayfish
The opalised gastroliths of freshwater crayfish are known locally as ‘yabby buttons.’ Crayfish use gastroliths to store calcium from their exoskeletons (‘shells’) before they moult, then release the calcium to harden their new protective coating. Fossil yabby buttons are usually around 10-12mm across Ref: http://www.australianopalcentre.com/opalised-fossils


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