Page "Gemstone" Paragraph 47

When jewelry containing diamonds is heated ( for repairs ) the diamond should be protected with boracic 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, it should not be coated with boracic acid or any other substance, as this can etch the surface ; they do not have to be " protected " like a diamond.