I had the shameful experience of breaking one of this rings. I got so scared from hearing about BeO that I mailed a bunch of safety hazard chemists and doctors. Most of them told me BeO is not used anymore in most kitchen appliances MWO and that it's most probably AlO which is enough to dissipate consumer devices' power. BeO was used in high wattage or military devices. After digging magnetrons manufacturer data sheets I found mentions of AlO and Chromium but no BeO.