Final answer:
Crystals have a complex shape due to the repeated crystal form at different orientations, resulting in X-ray diffraction and complex interference patterns.
Step-by-step explanation:
Crystals are substances in which particles are arranged in an orderly, repeating, three-dimensional pattern known as a crystal lattice. Crystal form refers to the external shape or morphology of a crystal. The crystal form is repeated at different orientations, giving the crystal a complex shape. This complexity arises from the fact that crystals can have many families of planes within their geometry, each one giving rise to X-ray diffraction. The resulting interference pattern can be quite complex.