 Not all solids are crystals.For example, when liquid water starts freezing, the phase change begins with small ice crystals that grow until they fuse, forming a "polycrystalline" structure.In the final block of ice, each of the small crystals (called "crystallites" or "grains") is a true crystal with a periodic arrangement of atoms, but the whole polycrystal does "not" have a periodic arrangement of atoms, because the periodic pattern is broken at the grain boundaries.