Crystals form by a process called nucleation, which takes place in solutions containing a high concentration of the solute. Once a nucleus is formed, the solute particles bind to it, thereby causing the growth of a crystal.
Crystals are usually formed when a highly saturated solution is prepared by adding high concentrations of a solute to a liquid, such as water, and dissolving the solute completely. If the dissolved solute molecules aggregate together, they form crystals, and this aggregation can be initiated by adding a nucleus to the solution. A nucleus is a solid surface that is added to the solution to provide the molecules a surface on which to aggregate, thus allowing a crystal to grow.
The aggregation of solute molecules can be spontaneously initiated in the solution without the addition of an external nucleus. The solute molecules are almost always surrounded or "shielded" by the solvent molecules. However, if there is a high concentration of the solute, the molecules can collide into each other and stick together to form a nucleus where crystals can grow. Most of the time, when the solute molecules collide, they have a tendency to break apart from each other rather than sticking together due to various forces present in the solution. However, if multiple collisions occur in a short time between several molecules, the aggregate of molecules can reach a critical size, forming a stable nucleus. Evaporation enhances crystal formation by removing the solvent molecules and allowing for increased frequency of collisions between solute molecules.