The crystallization process begins with nucleation, the initial step where a small number of particles (that is, ions, atoms, or molecules) come together to form a stable cluster of solid material. This cluster acts as a seed for further growth. There are two types of nucleation: heterogeneous nucleation—where a surface of some other substance becomes the site or location from which the cluster grows, and homogeneous nucleation—where particles come together uniformly within a medium or matrix, such as glycerol, as they randomly move through the medium and collide with one another. Heterogeneous nucleation is more common, with homogeneous nucleation becoming the more likely pathway in conditions of supersaturation (i.e., having too many molecules for the pressure and temperature conditions in the environment) or supercooling (i.e, cooling below the freezing point without solidification or crystallization).

Once nucleation has occurred, the crystal grows as more particles join the initial cluster. Growth, or accretion, can happen in a solution, in a liquid, or in a vapor. The particles continue to arrange themselves in a pattern characteristic of a crystalline solid. Temperature also plays a crucial role in crystal formation. For example, cooling a gas to a liquid and then to a solid can produce a polycrystalline solid (that is, any solid object made up of randomly oriented crystalline regions) unless special techniques are used. Methods like the Czochralski process, where a seed crystal is slowly pulled from a melt, allow for the growth of large, pure crystals. This method is particularly popular for growing silicon crystals used in semiconductors and computer chips.