The very definition of a fossil includes objects that have been replaced by another material. These are called casts. Impressions are also fossils, such as the shape of a prehistoric fern leaf embedded in shale rock or the impression of a shell in limestone. Somewhat common in Western Pennsylvanian fossils from the Carboniferous are shells that still contain the original material from which they were made.
These shells show up as a white color contrasted in the dark limestone locally. This is said to be preserved aragonite (CaCO3) shell material from the original shell. Being over 300+ million years from the present day, it’s quite remarkable that the shell material persists. Being a carbonite minteral, aragonite is one of three common carbonite minerals. Calcite and Vaterite are the other two that make up the group of three.