What this coin looks like (obverse, reverse, mint mark location, special features, etc.):
One major variety is the 1909/8 Saint Gaudens $20 Gold Double Eagle. This variety features a 1909 date struck over an 1908 date. This variety can easily be identified by observing the bottom-left portion of the second "9" in the 1909 date - parts of the underlying "8" is visible within the "9". Very subtle parts of the underlying "8" can be seen just to the bottom-right and top-left portion of the "9" as well. Depicted below is a closeup example image of the 1909/8 (9 Over 8) overdate:
Coin valuation chart: Typical coin prices, values and worth per grade or condition - in USD