I will give a hypothesis. Just like a tree, when you cut a tree by its trunk, you will the rings. The rings around it represents the tree's age. So when you cut rock, the pores in the rock determine whether it is an old rock or a fossil rock.
Explanation:
The reason is, if it is limestone and it is an old rock, the pores will be slightly big and if the limestone is a fossil rock, then the pores will be bigger due to leaching or denaturing of the minerals in the rock through the ages.
