The characterization and spectra–structure correlations of grossular and uvarovite garnets by Raman and infrared (IR) spectroscopy were carried out. Attention was focused on the explanation of the spectral differences between both minerals from structural and compositional points of view because they are isomorphous and share the general chemical formula [Ca3Y2(SiO4)3; Y = Al (grossular) and Cr (uvarovite)]. It was established that the prominent spectral differences are mainly due to the presence of various Y3+ cations and the possibility for their interchange. It was found that the substitution of Al3+ with Cr3+ promotes blue shifting of the high-energy Raman bands (SiO4 stretchings) and red shifting of the corresponding IR bands. This peculiar observation is explained by the introduction of two different effects that promote band shifting in opposite directions—the effect of different cation crystal field stabilization energies versus the effect of different cation masses and radii. Copyright © 2010 John Wiley & Sons, Ltd.