The solidus was first introduced by Diocletian
around 301 AD, struck at 60 to the Roman pound
of pure gold (and thus weighing about 5.5 g each) and with an initial value equal to 1,000 denarii
However, Diocletian's solidus was struck only in small quantities, and thus had only minimal economic effect.
The solidus was reintroduced by Constantine I
in 312 AD, permanently replacing the aureus
as the gold coin of the Roman Empire. The solidus was struck at a rate of 72 to a Roman pound
of pure gold, each coin weighing twenty-four Greco-Roman carats
or about 4.5 grams of gold per coin. By this time, the solidus was worth 275,000 increasingly debased denarii.
The solidus was maintained essentially unaltered in weight and purity until the 10th century, though in the Greek-speaking world during the Roman period and then in the Byzantine economy
it was known as the nomisma
Whenever the coin was taken in by the treasury, it was melted down and reissued. This maintained the evenness of the weight of the circulating solidi, since the coin did not tend to be in circulation long enough to become worn.