The purpose of deep boiling is to try and reach internal dirt, which the boiling materials can reach via a tiny crack known as an “open inclusion” the tiny bit of black graphite (“fix” as it’s known by diamantaires) trapped inside the diamond. These tiny dots are trapped in the diamond when it is formed. Since acid doesn’t corrode the diamond itself, but only the dirt that has entered the stone, the process allows dirt to be removed so the polished diamond looks better.