As they change to the solid state, the atoms or molecules join together in repeating patterns. Materials that do not form these patterns are called amorphous. Glass is a good example of an amorphous material. The usefulness of a crystal depends on its structure. All crystalline materials have varying degrees of defects.
Defects can take many forms.
In other crystalline materials, defects may actually enhance value. Crystals used for solid state electronics have impurities deliberately introduced into their structure that are used to control their electrical properties. Impurity atoms may substitute for the normal atoms in a crystal's structure or may fit in the spaces within the structure. Other defects include vacancies, where atoms are simply missing from the structure, and dislocations, in which a half plane of atoms is missing. The important thing about crystal defects is to be able to control their number and distribution.