These coins were minted in the ancient Roman empire in the provincial territory of Thracia governed by Markianopolis, located close to modern-day Devnya, Bulgaria.
The story of Diadumenian is a sad one. His father, Macrinus, ruled from 217 AD until he was defeated in 218. Diadumenian, aged 10, fled the province towards Parthia, but, just a child, was captured and executed.
These cool coins with the pentassarion denomination were minted in bronze when Diadumenian was age 9, in his joint reign with his father under the title of Caesar, later Augustus.
The coin in our picture is from seller Ancient Coin Art in California. They are on Vcoins.com, one of our favorite web sites for ancient coins. With its wear, it would be considered an average circulated coin.
These are very approximate values for these coins:
average circulated: $80
well preserved: $200
Each ancient coin like this stands on its own merits. Great eye appeal means a high value - you can guess what a tainted eye appeal would do to the value of a coin.
Beware of counterfeits. Only buy rare, valuable coins from sellers you trust.