This paper reports on the results of investigations into the phase composition of HgI₂ microcrystals formed in the bulk and on the surface of different nanocrystalline matrices: pressed, finely dispersed (with a specific surface of more than 210 m²/g) aluminum oxide powders; porous borosilicate glasses with pore radii ranging from 2 to 20 nm; and Al₂O₃-HgI₂ and SiO₂-HgI₂ nanocrystalline composites. The results of spectroscopic analysis (diffuse reflection, absorption, and luminescence spectroscopy) are complemented by the data of atomic-force microscopy (AFM) and x-ray diffraction. The experimental results indicate that nonequilibrium modifications, namely, the high-temperature yellow phase and the orange phase metastable in bulk crystals at all temperatures, can be stabilized in HgI₂ nanoparticles at room temperature.