More information is needed. Please post a new question giving the coin's denomination, condition, and if it's not a U.S. coin, the country that it's from.
U.S. pennies have never contained silver. Your coin is silver plated for jewelery or other decoration.
The U.S. did not mint any silver dollars in 1821. If you meant 1921, it's worth about $13 to $15 in average condition.If your coin is dated 1821, please check its denomination and post a new question. If it actually says it's a $1 coin it is a counterfeit.
Please look at the coin again. No U.S. silver dollars exist dated 1821.
No US silver dollars were made in 1821 look at the coin again, post new question
The value of a silver coin is always changing because the value of silver is always changing. See the related link below for a silver coin value calculator. This does not give the actual value of the coin but it does give the value of the metal used to make the coin. This is know as the melt value.
USD10k
20 bucks
Metal value depends on the purity of the silver and the weight of the coin. Numismatic value depends on the condition of the coin.
The US made NO silver dollars dated 1821, look at the coin again and post new question.
A Silver Eagle Bullion coin is 99.9% pure silver and weighs 1 oz per coin. The stamped value of the coin is U.S$1 however the intrinsic value of the coin is the value of the silver within that coin. If the the spot price of silver was $30oz at the time of checking, then the value of that coin would be worth at least $30 or more.
A denomination is needed, post new question
Therre were no 1821 British Double Sovereign or Two Pound coins minted.