Reference:Marble Pattern

The marble pattern is very similar to the gradient x pattern. The gradient pattern uses a default ramp_wave wave type which means it uses colors from the color map from 0.0 up to 1.0 at location x=1 but then jumps back to the first color for x > 1 and repeats the pattern again and again. However the marble pattern uses the triangle_wave wave type in which it uses the color map from 0 to 1 but then it reverses the map and blends from 1 back to zero. For example:

pigment {
  gradient x
  color_map {
    [0.0  color Yellow]
    [1.0  color Cyan]
    }
  }

This blends from yellow to cyan and then it abruptly changes back to yellow and repeats. However replacing gradient x with marble smoothly blends from yellow to cyan as the x coordinate goes from 0.0 to 0.5 and then smoothly blends back from cyan to yellow by x=1.0.

Earlier versions of POV-Ray did not allow you to change wave types. Now that wave types can be changed for most any pattern, the distinction between marble and gradient x is only a matter of default wave types.

When used with turbulence and an appropriate color map, this pattern looks like veins of color of real marble, jade or other types of stone. By default, marble has no turbulence.

The marble pattern has a default color_map built in that results in a red, black and white pattern with smooth and sharp transitions.

marble pattern used as pigment and normal respectively