The formation of the HgI₂ crystalline phase in pores of various porous matrices and on their surfaces was studied. Based on the analysis of the evolution of the HgI₂ optical spectra and electron and atomicforce microscopy data, it was shown that the formation of microcrystals of the stable red α modification is preceded by the formation and growth of particles of metastable yellow (β) and orange modifications. The absorption edge of the β phase at T = 77 K is formed by excitonic transitions with an energy of 2.7 eV. The Raman spectra of the β phase were compared to the spectra of known metastable yellow modifications.