This depends on 2 factors:


   1. The internal symmetry of the crystal, and

   2. The relative growth rates along the various directions in the interior the of the crystal.


For example, suppose you have mutually perpendicular axes, a, b, and c. Suppose the crystal grows at equal rates along a, b, and c, then the crystal shape will be a cube. Now suppose a different crystal grows fast in the a and b direction, but very slowly in the c direction. The crystal will then grow as thin plates with the face of the plate being perpendicular to c. These are only simple examples.


More complicated cases (and shapes) happen when the crystal doesn't have mutually perpendicular axes, and when the fastest directions of growth correspond to face or body diagonals (or even other directions) in the crystal.


Why crystals grow at different rates in different directions is a very complicated question. If there is a highly attractive interaction (energetically speaking) along a certain direction of a crystal, then that direction will probably grow fast. However, it could also grow slowly, if that direction interacted strongly with the solvent; having strongly absorbed solvent on the surface of the crystal could block growth along that face.

The color of any compound (whether or not it is a crystal) depends on how the atoms and or molecules absorb light. Normally white light (what comes out of light bulbs) is considered to have all wavelengths (colors) of light in it. If you pass a white light through a colored compound some of the light is absorbed (we don't see the color which is absorbed, but we see the rest of the light) as it is reflected off the surface. This gives rise to the idea of "Complementary Colors". If a compound absorbs light of a certain color the compound appears to be the complimentary color. Here is a table of colors and their compliments:

Color  Compliment  Wavelength

(of color nm)

violet  green-yellow  400-424

blue  yellow  424-491

green  red  491-570

yellow  blue  570-585

orange  green-blue  585-647

red  green  647-700


So if you have a crystal which absorbs red light, it will appear green. Conversely, if the crystal absorbs green light, it will appear red.