Simpler organic compounds, and even some viruses, are often crystalline in structure. Some solid materials, such as metals, salts, and powders, do not fit the common notion of crystals but do fit the scientific definition. The solids that fit the common notion of crystals are all large single crystals. In a single crystal the same orientation of the crystal pattern extends throughout the specimen. When this occurs, the symmetrical arrangement of the unit cells can show up in the overall appearance of the crystal.
Most common solids are polycrystalline.
Their molecules are arranged in a pattern, but a given orientation of the pattern extends over only a small area, called a grain. Each grain shows the symmetry that is observed in large single crystals. Growing Crystals at Home Originally, the study of crystal structure was limited to naturally occurring single crystals such as the precious and semiprecious gems. It is now possible to produce synthetic single crystals that are often larger and purer than natural crystals. They are prepared by various methods, all of which can use the process of growth from a seed.