Telling a nugget of gold from a material such as copper, which can be treated to closely resemble it, is not always easy. Chunks of "fool's gold," naturally occurring minerals such as iron pyrite, chalcopyrite or bismutite, also can deceive the unwary. There are, though, a number of straightforward ways of testing a suspect nugget to establish whether it truly is made of gold.
Hold the nugget in the beam of a desk lamp. Turn the nugget, so you can view it from various angles. The colour and lustre of true gold will remain constant, no matter how it is turned. Fool's gold, though, will show variations in its colour and in the brightness of its lustre.
Tap a small iron nail into the nugget, using a hammer. If the nugget cracks or any of it crumbles away, it is definitely not gold. Gold can be bent or dented, but it does not crack or crumble.
Hold a magnet close to the nugget. Gold will not attract magnets, so if the magnet is drawn to the nugget, the material you have cannot be gold.
Weigh the nugget and compare it with a heavy substance of a similar size, such as a stone. Gold is an extremely heavy metal, almost twice as heavy as lead, for example, so in most circumstances, a nugget of true gold should weigh more than another object of equivalent size. It will certainly weigh more than a copper nugget of the same size.
Drip a single drop of nitric acid onto the nugget. True gold will be unaffected, but a copper substitute will fizz a little and produce green foam. Nitric acid can be obtained from chemical suppliers or online retailers.