When a solid is hit hard, the force may break the solid. This is called.

Cleavage

Explanation:

Toughness is the resistance of a solid to breakage. The tougher a solid is, the less likely it would succumb to breakage of any form.

Hardness is very similar to toughness and it describes how unyielding a sold is to pressure or force of any form.

Cleavage on the other hand describes the tendency or ability of a solid to spilt or be broken along a specified plane of weakness. When such a solid is hit hard, they show some fracturing directions which represents weaknesses in their lattice structures or crystal forms. Most solids have cleavage directions in them.

This is called a cleavage