Reference:Agate Pattern

The agate pattern is a banded pattern similar to marble but it uses a specialized built-in turbulence function that is different from the traditional turbulence. The traditional turbulence can be used as well but it is generally not necessary because agate is already very turbulent. You may control the amount of the built-in turbulence by adding the optional agate_turb keyword followed by a float value. For example:

pigment {
  agate
  agate_turb 0.5
  color_map {MyMap}
  }

The agate pattern has a default color_map built in that results in a brown and white pattern with smooth transitions.

Agate as used in a normal:

normal {
  agate [Bump_Size]
  [MODIFIERS...]
  }

agate pattern used as pigment and normal respectively