Why are some crystals shaped like perfect cubes or hexagons?

A crystal grows when atoms or ions attach in an orderly, repeating structure called a lattice. Different minerals have different lattice symmetries, which leads to characteristic shapes such as cubes, hexagonal prisms, or rhombic forms. For example, halite commonly forms cubes, while quartz often grows in hexagonal crystals. External growth conditions also matter, so real crystals may be imperfect even when the internal structure is regular. Crystal habit is one of the most useful clues mineralogists use to identify minerals.