A crystalline substance is a solid substance composed of atoms or ions that are periodically repeated in the three-dimensional space. Crystalline substances are substances that contain crystals. All crystals spontaneously grow with the inherent characteristics of a geometrical polyhedron, but many crystals are limited by the external conditions during the growth process and therefore do not necessarily have the regular shape of the geometric polyhedron. When crystallisation starts earlier in a free space or when a mineral has a stronger crystallization growth ability, more perfect geometrical polyhedron, called euhedral crystals, would be formed. Some crystals do not have perfect geometric polyhedron shapes, and they are called subhedral crystals.