Reference:Facets Pattern

normal {
  facets [coords SCALE_VALUE | size FACTOR]
  [NORMAL_ITEMS...]
  }

The facets pattern is designed to be used as a normal, it is not suitable for use as a pigment: it will cause an error.
There are two forms of the facets pattern. One is most suited for use with rounded surfaces, and one is most suited for use with flat surfaces.

If coords is specified, the facets pattern creates facets with a size on the same order as the specified SCALE_VALUE. This version of facets is most suited for use with flat surfaces, but will also work with curved surfaces. The boundaries of the facets coincide with the boundaries of the cells in the standard crackle pattern. The coords version of this pattern may be quite similar to a crackle normal pattern with solid specified.

If size is specified, the facets texture uses a different function that creates facets only on curved surfaces. The FACTOR determines how many facets are created, with smaller values creating more facets, but it is not directly related to any real-world measurement. The same factor will create the same pattern of facets on a sphere of any size.
This pattern creates facets by snapping normal vectors to the closest vectors in a perturbed grid of normal vectors. Because of this, if a surface has normal vectors that do not vary along one or more axes, there will be no facet boundaries along those axes.