IDEAL CRYSTALS AND ACTUAL CRYSTALS
Most crystals found in nature are to some degree imperfect, as they usually stand on a surface, such as the wall of a druse, from which their growth started. The crystals which are completely perfect are the ‘floating’ crystals which grew in a soft medium, as, for instance, gypsum crystals in soft clay. The uneven supply of solution during crystallisation leads to distortions and generally the perfect growth of the crystal is prevented by the presence of adjoining crystals or the host rock itself. Due to such external influences, the shape of natural crystals frequently falls far short of the ideal shape. Apart from obvious structural distortions, there are other defects, such as small inclusions of other minerals, which may locally disrupt the orientation of the crystal lattice. Crystals of fairly large size are thus hardly ever completely uniform. Usually they consist of a mosaic of small blocks of homogeneous crystals which are variously orientated and may frequently interlock.