Nucleation [of crystals]
In the absence of container walls or of other objects that can act as seeds, liquids and gases can typically be supercooled quite far below their freezing points. It appears to be extremely unlikely for spontaneous microscopic fluctuations to initiate crystal growth, and natural snowflakes, for example, presumably nucleate around dust or other particles in the air. Snowflakes in man-made snow are typically nucleated by synthetic materials. In this case and in experiments on cloud seeding it has been observed that the details of seeds can affect the overall shapes of crystals that grow from them.