Your getting caught up in how a rock fits your hand, that is the marvel of the human hand, not the rock, any fist size rock will fit the human hand if you turn it enough times.
In my limited Knowledge..the rock characteristics are another clue.
A grinding stone has to stay “sharp” and have a “bite” as it wears down. An extremely hard stone would polish up and become very slick...which isn’t productive to grinding. It needs to stay abrasive.
Also it absolutely HAS to conform to the opposite stone it’s grinding against.
An irregular working face is a red flag regardless of how well it fits the hand.