The invention relates to a glass ceramic, wherein the initial glass comprises 30 65 percent (by weight) SiO.sub.2; 5 30 percent (by weight) Na.sub.2O; 5 30 percent (by weight) CaO, and 0-15 percent (by weight) P.sub.2O.sub.5, and wherein the main crystalline phases comprise alkali--alkaline earth--silicate and/or alkali silicate and/or alkaline earth silicate. The invention is characterized in that either a glass ceramic with a single crystalline phase 1 Na.sub.2O--2CaO--3 SiO.sub.2 is excluded, or the crystalline size of the glass ceramic is <10 .mu.m, or the weight fraction of SiO.sub.2 is <47%.