When it comes to testing the purity of gold, the penny test is a widely used method that is simple and easy to do. All you need is a copper penny and gold piece of jewelry. The first step is to file the surface of both the coin and jewelry piece so that they are both shiny. Next, place the gold piece on top of the penny with the shiny side up. If you see a dark line between the two pieces of metal, then your gold is not pure. This means that it is made with other metals and not just gold. However, if there is no line or only a very faint one, then your gold piece is most likely pure.