D is the mintmark for the Denver mint.
The "D" on the coin is the mintmark of the Denver Mint, NOT the state of Delaware. Average value of the coin is 5 to 10 cents.
You didn't provide the coin's denomination but if it's a gold coin, a small D would normally indicate that it was minted in Dahlonega, GA. The Dahlonega mint opened in 1838 to mint coins using gold from the recently-discovered deposits in that region. It closed in 1861 when the Civil War started.
Please post a new question with the coin's denomination. If you can find a small D or S mint mark on it include that detail as well.
It's called a mint mark and indicates where the coin was minted. The mint mark 'D' stands for Denver.
Only the 1970-S has the small date, not the 1970 or 1970-D. Retail values for circulated coins run from $9.00-$18.00 and mint state coins are $20.00-$57.00 depending on grade and color of the coin.
It is only gold plated. This adds no value to the coin. It is considered an altered coin and is worth 25 cents.
Looking at the reverse of the coin, to the left of the word cents between the rim and dot. It's very small and can only be a "D" or "S". It if doesn't have either a D or S, the coin was minted in Philadelphia.
The dime is a U.S. coin. It begins with the letter d.
Much more information is needed - please post a new question with the coin's date and the country that minted it. If it's a US coin, look for a small D or S mint mark.
If the coin came from circulation it's only face value. Just a high grade Mint State P or D coin is worth more than 50 cents.
A very common coin with retail values of $2.00-$3.00 for average circulated condition. Mint state coin is $4.00-$5.00...