Curves and Angles
I've always thought it's absolutely fascinating, the degree of complexity which emerges when water freezes. In particular, when it freezes slowly, at uneven temperatures.
Snowflakes are the best-known example of this, but frost patterns can be amazing too, in their richness and variety. All this arises from fundamentally quite simple behaviour of water molecules on a tiny scale - when they get cold enough to stick together, they do so in hexagonal patterns, because all else being equal that's the most efficient way to stack balls on a plane. This provides a seed shape, and self-catalysing processes scale it up from there.
Depending on the temperature, pressure and humidity, the growth might tend more towards plates or needles; when these factors changes while the ice crystals are still forming, an astonishly rich variety of forms becomes possible.