All atoms have a strong tendency to achieve a stable configuration of the outer electron shell in which all possible electron sites are filled. This is why bonding takes place. This means that the forces that bond crystals together are electrical in nature.
The type and intensity of bonding are responsible for the physical and chemical properties of minerals. Hardness, cleavage, fusibility (to become liquid or plastic), electrical and thermal conductivity, and thermal expansion are directly related to bonding forces. In general, the stronger the bond the harder the crystal, the higher its melting point, and the smaller its thermal expansion. Diamond is so hard because of the very strong electrical forces linking its carbon atoms.