Chrysoberyl, which is isostructural with olivine, has a low mean atomic weight M of 18.2 and a small molar volume of 34.3 cm3/mol. The single-crystal elastic constants in units of Mbar are (based upon a density of 3.72 g/cm3 and crystallographic axes b>c>a) C11-5.277, C22=4.387, C33=4.658, C44=1.444, C55=1.458, C66=1.518, C12=1.25, and C23=1.28, and C13=1.11. Calculated aggregate constants are bulk modulus λ=2.40 Mbar, shear modulus G=1.60 Mbar, compressional velocity Vp=14.10 km/s and shear velocity Vs=6.56 km/s. The elastic constant values of chrysoberyl do not fit into current velocity-density systematics of olivine structure compounds, even with an ionic size correction. The measured bulk modulus of Al2BeO4, however, is predicted to within 4% by taking the molar volume average of the bulk moduli of BeO and Al2O3. This method of averaging is successful because beryllium is in tetrahedral coordination and aluminum is in octahedral coordination in both Al2BeO4 and the oxides and because the average Be-O and Al-O bond lengths in Al2BeO4 are close to those in the oxides.