What is the hardest substance in the body? Tooth Enamel. What can make the enamel even harder? Fluoride. Fluoride is absorbed into the enamel and makes it a harder structure. This is beneficial because the plaque, which is bacteria, in your mouth produces acid which eats away at the enamel surface until it makes a hole in it called, "a cavity". The harder the enamel the more difficult it is for the acid to eat a hole in it. Fluoride for teeth is supplied in pill form, in municipal water supplies, in toothpaste and as a gel or liquid. Dental offices use higher Fluoride containing gels as a treatment at tooth cleaning visits. Good, hard teeth have a much greater chance of lasting a lifetime.