It is a resistance measurement of a material which is opposite of shape or deformation.
The hard materials are cement, metals, ceramics etc.
Diamond:
It is made up of carbon allotropes which are arranged in a specific cubic lattice.
Each carbon atom is attached to the other four carbon atoms through covalent bonds and these carbon atoms are arranged in a series to form a large structure with the help of a small unit of tetrahedra carbon atoms.
Material harder as compared to Diamond is:
The carbon nanotubes have the largest strength at the microscopic level.
Lonsdaleite is also an allotrope of a carbon atom which is considered a 58% harder material than the diamond because of the enhancement of bone density per unit area and it has a hexagonal geometry.
Boron nitride and Silicon carbide are the two other synthesized materials which are approx. 18% harder than the diamond but these are made up of different atoms.