What is this rock?

A Facebook friend posted these pictures wondering if anyone could figure out what this rock is.

He said, “Here’s the smooth river rock I found in my yard. Originally I am guessing there were two squares, one on the obverse and one on the reverse where the hole is The protrusion square has a 6 with three dots under it, most likely to assure people know it’s a six and not a nine. This stone is about a 4” x 3” oval, mixed black and gray. What the heck?”