Single crystals form only in special conditions. The normal solid form of an element or compound is polycrystalline. As the name suggests, a polycrystalline solid or polycrystal is made up of many crystals. The properties of a polycrystal are notably different from those of a single crystal. The individual component crystallites are often referred to as grains and the junctions between these grains are known as grain boundaries .
The size of a grain varies according to the conditions under which it formed. Galvanised steel has a zinc coating with visibly large grains. Other materials have much finer grains, and require the use of optical microscopy.
These photographs show a polycrystalline sample of quartz mixed with feldspar in which the grains all have optically anisotropic properties. Between the crossed polarisers, each grain allows transmission of light at a slightly different point in the rotation. This gives the strange effect seen here.