All crystals of the ice we encounter around us (Ice 1h – there are other types  stable under different conditions) have hexagonal symmetry.   We see it in six sided snowflakes and it is manifest in the halos of the skies.

Here, very slow overnight cooling has enabled a single crystal to grow over the bucket's water surface.  

The growth is dendritic along the directions of the three main ‘a’ axes of the fundamental hexagonal prism shape.

Why is ice hexagonal?   The symmetry arises from the lowest Gibbs free energy arrangement and bonding of four water molecules within a unit cell of the solid.    Ice crystals are built of stacks of these basic units like the bricks in a wall.   The shape of the brick and its stacking determine the ice macro-structure.