Abstract: Glass compositions having a Young's modulus of at least 16 million psi and a specific modulus of at least 110 million inches consisting essentially of approximately, by weight, 20-43% SiO.sub.2, 8-21% Al.sub.2 O.sub.3, 4-10% BeO, 27-58% of at least one oxide selected from a first group consisting of Y.sub.2 O.sub.3, La.sub.2 O.sub.3, Nd.sub.2 O.sub.3, Ce.sub.2 O.sub.3 and the mixed rare earth oxides and 3-12% of at least one oxide selected from a second group consisting of MgO, ZrO.sub.2, ZnO and CaO, the molar ratio of BeO to the total content of said first group oxides being from 1.0-3.0.