In some cases a second phase (second compound or element)  is present in the crystal in the form of inclusions (formed during growth) or precipitates (formed during the cooldown of the crystal)  They range from 0.1 to 50 micron in size and have various shapes.  Their presence can lower the optical transparency of the crystal or impede the electrical carrier transport through the crystal.   In metals they can improve the mechanical properties of the metal.