Individual fragments were loaded into the nozzle of an aerodynamic levitation furnace (see Supporting Information S1). An oxygen gas jet was used to levitate the samples, and a pair of CO2 lasers were used to heat them, with temperatures monitored by an optical pyrometer. Melting occurred at approximately 1800°C for all samples. The levitating melts were then held for a few seconds at ~2200°C, then quenched rapidly to ambient temperature by switching off the lasers, to yield transparent glass beads of ~2 mm diameter. The formation of glass was verified by PXRD of crushed beads, which showed no Bragg peaks. The glass-transition and crystallization temperatures were determined by differential scanning calorimetry. Crystallization was induced by heating glass powders or beads in a muffle furnace at 950°C in air for 3 hours, or for the same temperature and time in a reducing environment (5%H2 and 95% argon gas flow) for the Ce 3+ -doped series (Gd0.98Ce0.02)3+xAl5-xO12.