Why does your twenty dollar gold coin feel lite?

Answer 1

Its quite possible it is a fake.

To determine if your coin is a fake, first weigh it. It should weigh 33.431 grams or so. While naturally a heavily circulated coin will cause the coin to lose some weight, anything more than a slight variation should be immediately suspect.

Next, measure your coin compared to a known genuine piece, a coin that isn't made out of gold but is the right weight will likely either have too big of a diameter, or more likely too thick.

Also, look at the design of your coin, first, make sure your year and mintmark combination exists. If you have for example a 1915-D double eagle, you'd have a fake since no coin exists of that particular year and mintmark. Make sure your coin doesn't look "mushy" or look porous, those are hallmarks of cast coins (genuine coins are struck). Also, look at the edge when compared to a genuine coin, a Liberty Head double eagle should have a reeded edge while a St. Gaudens double eagle should have a lettered edge. While its possible that the reeding might not exactly match (especially if you are comparing coins from different years), you should make sure that your coin has the appropriate edge design for the type.

If all else fails, take it to a coin dealer and ask their opinion, most coin and bullion dealers deal with double eagles fairly regularly and can easily identify a fake.