Ceramic fibers comprising glass, and composites comprising such fibers. The glass comprises at least 35 percent by weight Al.sub.2O.sub.3, based on the total metal oxide content of the glass, a first metal oxide other than Al.sub.2O.sub.3, and a second, different metal oxide other than Al.sub.2O.sub.3. The glass contains not more than 10 percent by weight collectively As.sub.2O.sub.3, B.sub.2O.sub.3, GeO.sub.2, P.sub.2O.sub.5, SiO.sub.2, TeO.sub.2, and V.sub.2O.sub.5, based on the total weight of the glass. The first and the second metal oxides are each selected from the group consisting of Y.sub.2O.sub.3, REO, MgO, TiO.sub.2, Cr.sub.2O.sub.3, CuO, NiO, Fe.sub.2O.sub.3, ZrO.sub.2, HfO.sub.2 and complex metal oxides thereof. At least a portion of the glass may be converted to crystals.