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A Jura hole stone (or Jura Coral rock, or Jura cavity stone) is a natural stone found in the Jura mountains of France.
They are with light beige coloring and many cavities.
They come from the petrified sea deposits that were cause by tectonic shifts between mountains. The bizarre form and many cavities are caused by natural erosion over thousands of years.
These stones are protected : it is forbidden to pick them up. However, some are for sale for aquarium decoration.
References
Jura hole stone Wikipedia(Text) CC BY-SA