PURPOSE: To obtain thermally crystallizable glasses having high rigidity, high hardness, high strength, toughness, excellent thermal durability and chemical durability by making the glasses contain SiO2, Al2O3, NiO, MgO and TiO and/or ZrO2.

CONSTITUTION: The crystallized glasses of slabs or free circular disks having a nucleation temp. of 800 to 850°C and an average size of the crystals of <0.5 Mm in diameter after a heat treatment for 4 hours at 1,200°C are obtd. by compounding the respective batch components, expressed in weight % based on an oxide, of 40 to 60% SiO2, 10 to 35% Al2O3, 5 to 35% NiO, 0 to 20% MgO and 10 to 15% TiO2 in total and/or ZrO2, automatically tumble-mixing all these components, loading the mixture into a platinum crucible and melting the batch introduced into a furnace of about 1,600°C for 16 hours, then pouring the melt into a steel mold. These glasses are exposed for a sufficient time at 800 to 850°C to generate nuclei which are then exposed to 850 to 1,200°C. The glass-ceramic bodies growing a nickel spinelloid crystal on the nuclei are obtd.