2. Different approaches to hardness

2.1 "Diamond is the hardest material".

This statement raises two questions: is there a definition for hardness, and is the statement true? The term "material" refers to the combination of "matter" (in this case, carbon) and its associated "property" (hardness). As far as "matter" is concerned, carbon is one of the most common elements in nature, entering into its composition on all scales, from inert to living. It exists in several allotropic varieties (from the Greek allos = other and tropos = matter) that can be classified into two main categories: two-dimensional compounds (2D) and three-dimensional compounds (3D). The fullerene C ₆₀ , discovered some twenty years ago, can be considered "zero-D" even though the molecules, resembling...