The so-called hardness refers to the hardness of the object. Natural materials have high hardness and can be used to cut hard glass. 

Toughness is the ability of an object to return to its original state after deformation. Although diamonds are hard but have low toughness, they are easily broken. 

The hardness of plexiglass products is not high, but the toughness is good. The bending ability after heating is stronger than that of glass, but the surface is not hardened by tempered glass.