Ammonite

Ammonites are an extinct group of marine invertebrate animals, more closely related to living Nautilus species.
When polished some ammonit fossils display iridescence, a rare and valuable opal-like organic gemstone Ammolite(background).
An iridescent opal-like play of color is shown in fine specimens, mostly in shades of green and red; all the spectral colors are possible, however.The chemical composition of ammolite is variable, and aside from aragonite may include calcite, silica, pyrite, or other minerals.
A close-up...

...and a macro, where you can observe the crystals in the shell.