Plasma etching is emerging as reliable top-down self-masked approach for nanotexturing on a wafer-scale the surface of crystalline silicon. Differently sized and shaped nano-bumps can be obtained as a function of plasma parameters and, accordingly, different optical and optoelectronic effects have been already reported. In this work, we utilize a combination of techniques such as transmission electron microscopy, X-ray photoelectron spectroscopy, field emission gun scanning electron microscopy, in order to enlighten on structure, composition, and topography of these nano-objects. The results indicate that they present a composite structure, with a crystalline conic base and an amorphous extension with inclusion of metallic nano-crystals. These and other evidences suggest that the structures are partly formed by the sculpting action typically ascribed to the etching process, partly due to a deposition of amorphous composite matter, likely aggregated by a catalytic action of the metal particles.