The most common concept of hard and soft substance is
The most common concept of hard and soft substances is their relative resistance to indentation. Hardness is therefore a measure of the resistance to plastic deformation and is measured as a force per unit area of indentation. Hardness may be broadly defined as the resistance to permanent surface indentation or penetration.
Relative resistance to indentation
Relative resistance to abrasion
Relative resistance to fracture
All of the above