This denomination was first introduced in 214 AD by M. Aurelius Antoninus who was also known as Caracalla. When first issued this coin was worth two denarii. The coin started out at about 40% silver and declined over time. By the time of Gallienus, it turned into a bronze coin just a bit larger than the denarius, with little silver contnet. The coin later returned to its original size and was coated with a thin layer of silver called a 'wash', under Aurelian. Most coins available to collectors will look bronze as the silver has worn off. Those with the silver intact will sell for a higher price. Eventually the coins silver content declined to 5% before going out of circulation under Diocletian in 294 AD.