Crystals are formed by a process called nucleation. Nucleation causes certain atoms or molecules to dissolve into their individual units in a solvent. These molecules then connect with each other to form a nucleus. Eventually, the nucleus grows large enough to crystallize, and the remaining solute molecules attach to the nucleus until equilibrium occurs between the molecules in the crystal and in the solvent.
The process of nucleation can be either assisted or unassisted. Unassisted nucleation occurs when the process of crystallization starts with the molecules themselves, whereas assisted nucleation occurs on a solid surface. In assisted nucleation, solute molecules encounter a solid surface and attach to it.
Crystals can form in all parts of the world, from the surface to deep within the earth. They can even grow from vapors that are rich in mineral content. For example, sulfur often forms in volcanic areas due to the phenomenon of hot gases encountering cooler surroundings and depositing crystals. Crystals can also grow under other varying conditions. For example, crystals can form on rock walls, on other crystals or even on particles of dust. They can also grow quickly, slowly or stop growing altogether depending on changes in temperature and concentration of the solvent.