A tendency to assume crystalline forms is a property of all crystalloids ; its full or imperfect development depends on external circumstances.

Crystals produced in laboratories are generally inferior to natural crystals of the same substances, prohaby because of their too rapid formation. The faculty of crystallization is modified by specific dispositions inherent to the substances. There are many distinct types of crystallization, connected with each other by intermediate types, and even with a transition from amorphous to crystallizable substances.

The degree of distinctness of crystals constitutes a fundamental difference. The crystals of many substances are distinct, while below a certain limit of size ; but when this is exceeded, the surfaces are interrupted by re-entering angles, or by irregular drusy cavities or excrescences constantly re-appearing when cleft away, so as, at last, to completely destroy their individuality and regularity.