A Stone – Corundum is made of a stable bond which contains pure clay, AI₂O₃, SiO₂, Fe₂O₃, CaO, Na₂O₃, K₂O₃ etc. and abrasive grain and hardened by heat at 1,300˚C – 1,400˚C. The specific gravity is 2.5 – 3.0.