Ammonite
Septa detail in an ammonite.
Septa are the walls of the chambers of the shell. They were filled with seawater or air to change elevation. 
The process of fossilization is done through mineral replacement. Pyrite can replace the septa of an ammonite, as seen above. This ammonite is likely to have been treated with bismuth, causing the rainbow color of the crystals. 

Ammonite


Septa detail in an ammonite.

Septa are the walls of the chambers of the shell. They were filled with seawater or air to change elevation. 

The process of fossilization is done through mineral replacement. Pyrite can replace the septa of an ammonite, as seen above. This ammonite is likely to have been treated with bismuth, causing the rainbow color of the crystals.