Final answer:
The hardest minerals are usually held together by covalent bonds.
Step-by-step explanation:
The hardest minerals are usually held together by covalent bonds.
For example, diamond - the hardest known substance - is a giant molecule of carbon atoms held together in a 3D network of covalent bonds. Other covalent network solids like silicon dioxide (sand) and silicon carbide (carborundum) are also hard materials.
Covalent network solids are usually hard because the covalent bonds that hold them together are relatively strong, requiring a large input of energy to break or melt the substance.