The composition of garnet inclusions in natural garnets is exceptionally useful for identification of the chemical and physical properties of mantle lithologies in which they occur. The results of Raman study of garnet inclusions in Siberian diamonds are presented. Most of studied garnets belong to either peridotitic (P type, n = 101) or eclogitic (E type, n = 63), and some rare garnets are websteritic (n = 5). Detected spectroscopic features of garnets in diamonds reveal a wide wave number ranges for rotational R[SiO4], bending ν2, and stretching ν1 modes. The peak positions of the Raman bands correlate with content of major components of garnets. Raman shift of stretching ν1 and bending ν2 vibrations reflects Ca–Mg and Al–Cr isomorphism. The found correlations have been applied for rough estimation of the garnet compositions. This method allows to discriminate E-type and P-type garnets marking diamondiferous mantle rocks.