This paper presents a new viewpoint that the differential diffusion velocities control crystal forms in accordance with the different influences of crystallized ions in the course of mineral crystallization, and that the generation of differential diffusion velocities is attributed to the reason that the concerted effects of external factors (consistency, temperature, pressure, pH, viscosity etc.) cause the differences in diffusion velocities of different ions and that the differential diffusion velocities of different ions lead to produce occupancy selection and dominant net in the course of mineral crystallization. The crystal form results from concerted effects of all external factors and the result, in turn, can reflect physical_chemical conditions for mineral crystallization. It is the differential diffusion velocity of each ion that controls occupancy selection and leads to form dominant net and then form crystal shape that can reflect the differential diffusion velocity of each ion. If we probe into morphological typomorphic characteristics of crystals and study physical_chamical conditions of mineral crystallization by ascertaining dominant net and analysing external factors that affect diffusion velocity, we can comprehensively consider all influences of external factors and avoid partiality enumerated in this paper. Furthermore, morphological typomorphic characteristics of such minerals as pyrite and fluorite are surveyed on the basis of differential diffusion velocity. In addition, the manner of growth of crystal and relationship between relative intensity of powder diffraction and dominant net are discussed in this paper.