We demonstrate the fabrication of acoustically engineered diamond nanoparticles-based metamaterials and their internal microstructure characterization using x-ray microcomputed tomography (XμCT). The state-of-the-art technique based on the radiation force of ultrasound standing (or stationary) waves in a rectangular chamber is utilized to pattern clusters of 5-nm-diameter diamond nanoparticles in parallel planes within a three-dimensional (3D) matrix of epoxy before solidification. Gradually, the periodic pattern becomes permanent with full cure of the epoxy matrix so as to form a 3D metamaterial structure. We also show that the periodicity of the pattern can be changed by selecting a different ultrasound frequency. Furthermore, XμCT is used as a quality control tool to map the internal structure and characterize each metamaterial. The ultimate application is to use the results as a base for the development of finite-element models which take into account all the structural features to study the various metamaterial (optical, acoustical, thermal, etc.) functional properties.