Smectic, or multilayer, structures in dry sol sediments of beta-FeOOH (ferric oxyhydroxide) have been investigated using an atomic force microscope (AFM). The AFM technique has provided three-dimensional information of the smectic structures in a nondestructive manner. The shading in the AFM images of the dry sol surfaces evidently shows that in the iridescent (red, orange, yellow-green, green, and blue) regions, the beta-FeOOH crystals are standing upright at a tilt (with respect to the plane of the smectic layer) and form approximately a square lattice. In contrast, in the noniridescent (gray and brown) regions, the crystals lie parallel or randomly oriented to each other. By determining the tilt angle of the crystals in the layer and the refractive index of the layer, we have found the thickness of the layer to be close to the wavelength of visible light in the layer. This supports an earlier periodic-spacing model that attempts to explain the origin of the iridescent colors. The AFM images also show that the individual crystal surface is homogeneous crystallographically, thus disputing an earlier bundle-structure model of individual beta-FeOOH crystals. Furthermore, the AFM images show that the crystals have stepped surfaces, tapered ends, and either convex or concave ends, thus indicating nonuniform crystal growth. The dry sol surfaces of beta-FeOOH have been further investigated with a scanning electron microscope (SEM) on a larger scale than is used with AFM. In the SERI images, we have found the following : (1) The in-layer structure of the smectics is, at least locally, square. Although defects destroy the two-dimensional crystalline order, a longer-ranged bond-orientational order still exists. (2) The smectic structures exhibit bending of the directors (which is usually observed in nematics) and overall layer undulation. We have also observed the coexistence of several smectic domains, each of which has a distinct orientation, and mono- and several-folded layers confined in the sols. The smectic structures of dry beta-FeOOH sols have been found to be obtained not only from suspensions with attractive-force interparticle interactions but also from those with repulsive-force interactions.