When jewelry containing diamonds is heated (for repairs) the diamond should be protected with boric acid; otherwise, the diamond (which is pure carbon) could be burned on the surface or even burned completely up. When jewelry containing sapphires or rubies is heated, those stones should not be coated with boracic acid (which can etch the surface) or any other substance. They do not have to be protected from burning, like a diamond (although the stones "do" need to be protected from heat stress fracture by immersing the part of the jewelry with stones in the water when metal parts are heated).