Minerals and Metals in Ancient India v2.c43.s3.t2.4

v2.c43.s3.t2.4

Sarma (1984: 51-52) has expressed surprise in the above statements since ruby and sapphire have the same Al₂O₃ lattice as corundum, and are supposed to have similar hardness. However trace substitution by low energy oxides of Cr, Fe etc. might lower surface free energy and hardness of Al₂O₃ by a small amount, and this may be detectable when pure corundum powder is mounted on a grinding wheel. Bauer (1968: 282) has reported slight differences in hardness values of the varieties of corundum, and the fact that ruby is slightly softer than sapphire.