The very definition of a fossil includes objects that have been replaced by another material.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.

Aragonite preserved within limestone.
Microscope photo of preserved aragonite shell material exposed while splitting limestone.
Electron microspope image of Aragonite layers.
Scanning electron microscope image of aragonite layers in the nacre of a blue mussel (Mytilus edulis). By Glenn Elert
Brachiopod Umbo, preserved in original Aragonite.
What appears to be the Umbo of a Brachiopod, with Aragonite preserved.

More about Aragonite Online