The 1899 US coin with an eagle on the back is the $20 gold double eagle. It is made of 90% gold and 10% copper, and weighs 33.44 grams. The obverse of the coin features the head of Liberty facing to the left, surrounded by 13 stars (representing the original 13 colonies). The reverse of the coin features a majestic bald eagle with outstretched wings, perched on a rock. The coin has a reeded edge.
The $20 gold double eagle was first minted in 1849, and was produced until 1933, when President Roosevelt issued an executive order to recall all gold coins. The gold double eagle was used extensively in international trade, and was also popular among U.S. citizens as a form of savings.
The value of an 1899 $20 gold double eagle depends on several factors, including its condition, rarity, and the current gold prices. In general, an 1899 $20 gold double eagle in good condition can be worth anywhere from $1,500 to $2,500.