Alien egg!

This rock appears to be a type of fossiliferous limestone. This type of rock is characterized by the presence of fossils, which are visible as various shell-like structures embedded within the rock. Limestone is primarily composed of calcium carbonate and is commonly found in marine environments where the accumulation of shell debris from marine organisms occurs over time. The texture and color variations in the rock suggest it might have undergone some weathering or diagenetic processes, which are common in such sedimentary rocks.