In as-sintered silicon nitride with yttria as a sintering aid, the intergranular glassy phase contains Y, Si, O, and N. Results of crystallization of partly crystalline, as-quenched melt samples containing 28–40 mol% Y2O3 in the Y2O3–SiO2 system are presented. Three different compositions were melted in tungsten crucibles at 2100 °C in 1 and 50 atm nitrogen and heat treated in air in the temperature range of 1200 to 1600 °C. Phase identification by x-ray diffraction (XRD) and transmission electron microscopy (TEM) showed crystallization to β, γ, and δ polymorphs of Y2Si2O7.