Halo phenomena are produce by sun or moon light which is reflected or refracted thought airborne ice crystals. Shape of the ice crystal plays very important part in halo formation process. 

Here we have three main variation of the basic shape of a hexagonal ice crystal. Namely a plate ice crystal, an equidimensional ice crystal and a column ice crystal.

They are all only a variation of the basic hexagonal (6-sided) form of ice crystal. What makes these variations important is their different orientations when they are floating in the air. Plate ice crystal is usually oriented its basal faces horizontally as it is in this image. Column ice crystal is also horizontal, but it is its principal axis, namely c-axis, which is horizontal. Equidimensional ice crystals are thought to be randomly oriented when halo simulations are made. Are they randomly oriented in the nature is an open question.

Most significant variation of hexagonal ice crystal is so called pyramidal ice crystal. It is a hexagonal ice crystal too, but it has pyramidal ends in one or both end of the crystal. Pyramidal ice crystal produce a whole family of rare halos, name odd radius halos.

It is sometime hard to remember, that the hexagonal ice is in matter of fact a mineral. It is a mineral which has the same hexagonal symmetry as beryl and apatite have. 

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