the stone may have been an early historic food preparation metate type object, or it could have even been used in some sort of crude early smelting operation.
do you see any marks in the depression area?
On the perimeter of the stone, are there any peck marks? Peck marks are left when shaping the object through repeated hitting of the stone with another object, which could have been another smaller and harder stone. In Indian Artifacts a pecking stone is used to shape other hard stone relics, such as axes and celts. On the surface the use polish often smooths the surface but small peck marks sometime remain.
Any which may have been made when the grinding took place which may have created the depression?
What is the dimension of the object and the dimensions of the depression?
Not sure what the scale is, but the top stone looks like a building block from a pier. The "mortises and tenons" would lock the stones together.
The other one looks like it's been used to sharpen tools of some kind.