This is a limestone pebble that contains many embedded fossil shells from the Pliocene and Miocene periods (approx. 5-23 million years ago). This piece was found in a beach area known for it's expansive Miocene-era deposits.
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.