The focus of this package is the difference between single crystals, polycrystals and amorphous solids. This is explained in terms of the atomic scale periodicity: single crystals are periodic across their entire volume; polycrystals are periodic across individual grains; amorphous solids have little to no periodicity at all.
The different atomic structures can have effects on the macroscopic properties. A single crystal may exhibit anisotropy - we have seen mechanical anisotropy of gypsum, and optical anisotrpy of quartz. Polycrystals may also be anisotropic within each grain, as seen when the polycrystalline quartz-feldspar mix was placed between the crossed polarisers. Amorphous solids do not have anisotropic mechanical or optical properties, since they are isotropic on the atomic scale.
Defects may exist in all structures, even single crystals. They include vacancies and grain boundaries, where the regular repeating structure is disrupted.