While reading the back of a tube a toothpaste [Underling] noticed that one of the ingredients was hydrated silica, gears turned, sparks flew and he wondered if he could possibly make a transistor out of the stuff. After thinking about it he decided that making a diode out of toothpaste would be easier and still prove the idea.
The quick n dirty explanation of this is he smeared some toothpaste on a bit of chrome and set it on fire with a propane torch. When set on fire the result is silica and sodium, heat causes the sodium to bond with the silica and since sodium is negatively charged this forms an n-type semiconductor or half of the diode. Chrome is used for the second half of the diode, for a few reasons, he had some lying around, its positivity charged, and the toothpaste contains a little bit of lye which oxidizes the chrome and burns off, bonding the silica to the metal.
What is left is a thin layer of chrome doped silicon under a layer of sodium doped silicon, which in spots where everything is perfect, acts like a diode, blocking current in one direction but not the other.