Bivalve Fossil (Internal Mold)

Hardness: 3-4 (if calcitic), Color: Grayish-brown with iron staining, Luster: Dull/Earthy, Crystal Structure: Typically microcrystalline or replaced by matrix, Cleavage: None (conchoidal fracture on matrix)

Formed when the soft tissue of a prehistoric mollusk decayed, leaving a hollow shell that filled with fine sediment (silt or mud). Over millions of years, the sediment lithified into stone while the original shell often dissolved, leaving an 'internal mold'. Commonly found in marine sedimentary layers from the Paleozoic to Cenozoic eras.

Used primarily for educational purposes, scientific research into paleoecology, and as a popular entry-level specimen for hobbyist collectors.

Bivalves are one of the most successful groups of marine animals and have survived several mass extinctions. The 'lines' or ridges seen on the specimen represent the growth intervals or the impression of the original shell's interior anatomy.

Identify in the field by looking for a symmetrical 'heart' or 'clamshell' shape in limestone or mudstone outcrops. They are common in areas that were once prehistoric seafloors, such as the Midwestern United States or the Jurassic Coast of the UK.