Certain minerals usually break along flat surfaces while other minerals break unevenly the characteristics is due to the

Cleavage of the mineral causes breakage of minerals unevenly along flat surfaces.

Explanation:

Cleavage refers to breaking of flat surface minerals into many surface. The crystalline structure of mineral determines the cleavage. It is the ability of crystalline materials to divide into “definite crystallographic structural planes”.  

The permanent locations of atoms and ions in crystal are the main reason for the relative weakness, which forms smooth repeating surfaces. They are visible both in the microscope and to naked eye.