It's impossible to definitively say what a "hard and rock-like fossil" is without more information. This description could refer to many different types of fossils!

To help you figure out what you're looking for, could you tell me more about the fossil? For example:

* What shape is it? Is it round, flat, long, or something else?

* What size is it? Is it tiny, small, medium, or large?

* What color is it? Is it brown, gray, black, or another color?

* Where did you find it? Was it in a rock, on a beach, or in a museum?

* Do you have any other details about it? For instance, does it have any markings or patterns?

With more information, I can help you identify the type of fossil you're looking at.