Ammonite
Ammonites are extinct cephalopods that existed from the Devonian Period to the Cretaceous Period. The most recognizable ammonite form is of a curled shell, although many ammonites don’t have the prominent curling pattern. As soft body tissue is nearly impossible to preserve for millions of years through fossilization and weathering, the shell is the main part of the ammonite that remains somewhat intact. 
Mineral replacement, perhaps along with titanium or bismuth treatment, created the rainbow crystals above in the septa of the ammonite.

Ammonite

Ammonites are extinct cephalopods that existed from the Devonian Period to the Cretaceous Period. The most recognizable ammonite form is of a curled shell, although many ammonites don’t have the prominent curling pattern. As soft body tissue is nearly impossible to preserve for millions of years through fossilization and weathering, the shell is the main part of the ammonite that remains somewhat intact. 

Mineral replacement, perhaps along with titanium or bismuth treatment, created the rainbow crystals above in the septa of the ammonite.