In this work diffraction of light and x-rays (XRD) on opal-like structures (OLS) is considered. New methodological approaches to collection, processing and interpretation of experimental data are presented. The microradian XRD reveals the type and degree of imperfection of opal-like structures. It was shown that the OLS ordered in the twinned face centered cubic (FCC) structures with the lattice constant of 835 ± 10 nm but also with clear tendency to the random hexagonal close-packed (RHCP) structure along the [111] axis. A novel representation of the light diffraction data in the "incident angle-scattered angle" coordinates θ, Θ allows one to easily distinguish the reflections originating from 2D diffraction from the ones governed by 3D Bragg diffraction. Structural disorder becomes apparent in the θ, θ representation, as well. In addition, it was demonstrated that the immersion spectroscopy method can be used to selectively switch diffraction reflections. This phenomenon is caused by optical inhomogeneity of the SiO₂ particles that form opal-like crystals.