I have found information that after special treatment corundum has tensile strength 2 GPa. Can you confirm this information? Who may manufacture such corundum?

Unfortunately, we didn`t hear about it anything. But we think it is not seriose info, because leucosapphire is not very plastics material, but more soon brittlen.

But if the sapphire is annealed at around 1850C for 24 to 48 hours, preferablly in O2, but air will do, the tensile strength goes up (even though Brittle materials do not have a yield point (are not plastic) they still have a Tensile strength). In particular, this anneal heals many micro defectds that would cause the breaking strength value. Annealing at these temperaturers also “fire polishes” ground surfaces, in effect passivating the surface, and elliiminating crack propication sights.