Stones - Agate/Onyx - Fossil
Fossil agate is a general term for instances where various types of agate have formed as mineral replacements in fossilization. While some specimens are extraordinary, many are just concretions of small shells that form an interesting marble-like pattern. You can often pick out the shapes of different shells in large chunks. The concretions are sometimes cut for jewelry.
Oval cabochon of fossil agate. Shell forms can clearly be seen in this specimen.