Mohs scale of mineral hardness
Some tension rings exert up to 50,000 psi (350 MPa) on the diamond. It is not possible to exert so great a pressure on a stone with a Mohs scale of mineral hardness rating of less than 9.0. Furthermore, to exert this pressure on a gem, particularly one of high importance, the maker must ensure even pressure distribution. This requires careful calculation, and inspection of the gem to ensure that every facet mates perfectly with the ring.