Ok, in the Hayabusa Ninja Village, there is a stone with a crack in it, and when you "Interact" with it, it says;
"To widen this crack, a heavy blunt object is needed"
Where can I find such an object?
Also, there is one in Tairon, but I can't "Interact" with it.
Anyone know what these cracks are for?

Patience... the animation after beating Alma will show you where to find your first heavy weapon. Then you can start smashing cracked walls in Tairon. You won't be able go back to the village to break the stone before chapter 13, though.

kill alma then after that gay cutscene of doku taking alma to his secret hideout with thse big flashy neon lights go back in the temple and look around and youll find that big hammer
(its totally crap in fighting until level 2)