Answer:The ones that are tightly bound together look different than do ones with looser bonds. For example, in the illustrations below, (A) shows the atomic structure of carbon in a diamond, and (B) is the carbon arrangement in graphite. (A) is more compact than (B), hence it is harder.
Explanation:Rocks are made of minerals, and different minerals have different crystal structures. The chemical bonds that hold atoms together in these minerals are stronger in some than in others, and the atoms themselves determine which bonds are stronger than others.