Page "Crystal" Paragraph 13

The flat faces ( also called facets ) of a euhedral crystal are oriented in a specific way relative to the underlying atomic arrangement of the crystal: They are planes of relatively low Miller index.

This occurs because some surface orientations are more stable than others ( lower surface energy ).

As a crystal grows, new atoms attach easily to the rougher and less stable parts of the surface, but less easily to the flat, stable surfaces.

Therefore, the flat surfaces tend to grow larger and smoother, until the whole crystal surface consists of these plane surfaces.

( See diagram on right.