In this study the relations among nonbridging oxygen (NBO), optical properties, optical basicity, and color center formation in CaO–MgO aluminosilicate glasses were studied. Samples containing (in mol  % ) 35.9–57.5 of CaO, 16–27.7 of Al ₂ O ₃ , 7.9–41.6 of SiO ₂ , and 6.5–6.9 of MgO were measured by optical absorption and excitation, luminescence, and Raman spectroscopy. The results showed that when the SiO ₂ content was increased, the absorption edge shifted toward lower wavelengths and the bonds between O ^2- ions and cations became more covalent. These observations were confirmed by Raman results that showed a decrease in the number of NBO per silicon tetrahedron as a function of SiO ₂ content. The results indicate that the effects of higher NBO concentration are the narrowing of the band gap energy and the delocalization of O ^2- electrons, which facilitates the O ^2- electrons to be trapped by anion vacancies and, consequently, forming color centers. The relationship between color center formation and SiO ₂ content was confirmed by optical spectroscopic measurements under UV radiation.