A hard substance deposited on the surface of the crown or root of a tooth; tartar. It is formed by calcium and phosphate salts that are dissolved in the saliva that precipitate on the teeth. An example of precipitation: If you take salt and put it in hot water it dissolves. If you cool the water down then the salt reappears. That's what precipitation is.
This is why you need to have your teeth cleaned, because you can't get this off yourself. Dental instruments need to be used. If the calculus is not removed, plaque bacteria can get bound up in the calculus and this can result in gum disease and tooth decay.
