Single Crystals
Crystals can be single crystals where the whole solid is one crystal. Then it has a regular
geometric structure with flat faces.
Polycrystalline Materials
A solid can be composed of many crystalline grains, not aligned with each other. It is called polycrystalline. The grains can be more or less aligned with respect to each other. Where they meet is called a grain boundary.
Non-Crystalline Solids
In amorphous solids, there is no long-range order. But amorphous does not mean random, since the distance between atoms cannot be smaller than the size of the hard spheres. Also, in many cases there is some form of short-range order. For instance, the tetragonal order of crystalline SiO2(quartz) is still apparent in amorphous SiO2 (silica glass.)
