As mentioned before, the apparent smoothness of a material is a function of how strongly it reflects and the size of the specular highlight. This is affected by the overall magnitude of the GL_AMBIENT, GL_DIFFUSE and GL_SPECULAR parameters, and the value of GL_SHININESS. Here are some heuristics that describe useful relationships between the magnitudes of these parameters:
No promise is made that these relationships, or the values in Table 3 will provide a perfect imitation of a given material. The empirical model used by OpenGL emphasizes performance, not physical exactness.
For an excellent description of material properties, see  for more information.
Table 3: Parameters for common materials