This is a limestone-based concretion that contains many embedded fossil shells from the Pliocene and Miocene periods (approx. 5-23 million years old). This piece was found in the Peace River (Florida, Bone Valley formation, Hawthorn group).
Refer to the photo. The black centimeter cube is shown for scale and is not included. You are purchasing the specimen shown. It measures approx. 1 inch x 1.25 inches at it's widest point.