Crystal formation happens in two stages:

• Nucleation — a few atoms or molecules come together to form a tiny seed. This is the hardest part, because it means overcoming an energy barrier.
• Growth — once the seed exists, more atoms attach to its surface, extending the lattice layer by layer.

Two conditions shape the result. Supersaturation gets things started: the solution holds more dissolved substance than it normally can, forcing crystals to form. And slow cooling yields large, well-formed crystals, while rapid cooling gives many tiny ones or even an amorphous solid like glass.