Fossil pyrite Kosmoceras ammonite of the Volga River formation, found between the towns of Ulyanovsk in the north and Saratov in the south produces some of the best pseudomorph pyritic ammonites known. The ammonite is replaced with pyrite after calcite. The whole ammonite has changed its appearance as the pyrites replaced the shell and tissue of the cephalopod. In some specimens inner chambers display rainbow pyrites wonderfully.
Pyrite is a form of Iron Pyrite, commonly known as fools gold, there is naught foolish about admiring these gems of the earth and every reason to hold them precious even though they are not as costly as gold. We like to think of them as fossil gold.
Ammonites are part of the Ammonitida of marine invertebrates. These were fascinating creatures from the deep oceans and believed to be aggressive creatures like squids today. With extremely large eyes and remarkable vision they could easily hunt on their prey at great depths where little or no light penetrates the deep oceans. The Cephalopod would attain its prey using their long tentacles, prey such as other crustaceans and fish, much as the persisting modern day Nautilus Pompilius which can be found in the Pacific today.