There are a lot of metals that on the surface look similar to gold. Centuries ago, people discovered that unscrupulous operators would take advantage of this to trick people into paying for worthless metal. In order to determine whether gold was genuine or not, scientists devised an “acid test.” The item that is supposed to be gold is rubbed on a black stone, leaving a mark behind. Gold is what is called a noble metal, meaning that it is resistant to the corrosive effects of acid. If the mark is washed away by the acid, then the metal is not real gold.