The degree of crystallinity has a large influence on hardness, density, transparency and diffusion.
In an ideal gas, the relative positions of the atoms or molecules are completely random.
Many materials, such as glass-ceramics and some polymers, can be prepared in such a way as to produce a mixture of crystalline and amorphous regions.
For instance, most metallic alloys are crystalline, but they usually comprise many independent crystalline regions (grains or crystallites) in various orientations separated by grain boundaries; furthermore, they contain other crystallographic defects (notably dislocations) that reduce the degree of structural perfection.
The most highly perfect crystals are silicon boules produced for semiconductor electronics; these are large single crystals (so they have no grain boundaries), are nearly free of dislocations, and have precisely controlled concentrations of defect atoms.