What factors controls the shape of a well formed crystal?

The following are major factors that influence the shape of a well formed crystal:
 

•   Symmetry properties of the underlying lattice
•  Rates of growth; - the overall shape of the crystal is determined by the relative rates of growth of the various faces. Thus the slower the growth rate, the larger the face. 
• The conditions under which a crystal is grown can affect its habit. The Temperature, degree of super-saturation, nature of the solvent all have their effects, and these may affect the growth of different faces in different ways. 
• The presence of impurities in the solution can radically alter the habit of a crystal, as seen in the following table for the growth of sodium chloride: 

no impurity           Fe(CN)64–         formamide Pb2+,             Cd2+          polyvinyl alcohol
cubes                     dendrites                octahedra              large crystals          needles

These effects presumably come about because these substances preferentially adsorb to certain faces, impeding their growth.