What metal was used in a 1933 double eagle copy?

It is impossible to say. Since copy coins are not made by the US mint and instead by private companies, they use any number of materials to make the coin. It is possible you have a pewter or copper coin that is gold plated, it is also possible that you might have a better copy that is made out of the 90% gold that genuine coins were made out of, it is also possible that you might have a brass copy, etc. If you are unsure of the composition, try taking it to a coin dealer or jewelry shop and they can usually tell you what your coin is made out of.