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.
[1] 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,
[2] 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 (plural
nomismata).
[2] 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.
[2]