    This pattern is very beautiful and similar to marble, but uses a differ-
 ent turbulence function.  The turbulence keyword has no effect, and as such
 it is always very turbulent.  You may control the amount of the built-in
 turbulence by adding the "agate_turb" keyword followed by a float value.
 For example:
          pigment {
             agate
             agate_turb 0.5
             color_map {
                ...
             }
          }
