Ostwald studied the crystallization behavior of solids, especially those solids that are capable of crystallizing in different forms, in the phenomenon known as polymorphism. He discovered that solids do not necessarily crystallize in their most thermodynamically stable form but instead sometimes crystallize preferentially in other forms dependent on the relative rates of crystallization of each polymorphic form. Ostwald found that the relative rates were dependent on the surface tension between the solid polymorph and the liquid form. Many common materials exhibit this type of behavior, including minerals and various organic compounds. This finding came to be known as Ostwald's rule.