Crystallization is the (natural or artificial) process by which a solid forms from solution, where the atoms or molecules are highly organized into a structure. Some aspects of the process, particularly nucleation have still not been completely described, despite intensive research going back more than a century.
“Crystallization has been called “one of the most secretive processes in nature”. The mystery lies in how structures with long-range order form from building blocks that only interact with their local neighbors. The most extreme case is when solids form from weak solutions, in which the molecules begin widely separated so that not even local structure is present.”
“One of the most important insights from the recent work has been the discovery of nonclassical pathways for crystallization. The naive picture of crystallization is that a few molecules come together by chance and happen to be arranged in a crystalline form. Additional molecules attach one by one, gradually building a larger structure. Small clusters are unstable because most molecules are near the surface and do not have the correct number of neighbors, but large enough clusters are stable.”
Source: How crystals form: A theory of nucleation pathways in Science Advances, Apr 2019: Vol. 5, no. 4,