Short answers, pulled from the story.
Sestertius means "two and one half", referring to its nominal value of two and a half asses. Latin writers derived the name from semis ("half") and tertius ("third"), because two asses plus half of a third as equals two and a half.
The sestertius was introduced around 211 BC as a small silver coin worth one quarter of a denarius. It was reintroduced in or about 23 BC by Augustus as a large brass denomination after the silver version ceased production around 44 BC.
Republican sestertii were silver. The imperial sestertius introduced by Augustus was made of brass, which Romans called orichalcum or aurichalcum, a word that reflected its gold-like color when newly struck. Later sestertii became darker as repeated melting reduced their zinc content.
Price notes from Pompeii in AD 79 record one modius (about 6.67 kg) of wheat at seven sestertii, rye at three sestertii per modius, a tunic at fifteen sestertii, and a donkey at around five hundred sestertii. A loaf of bread cost roughly half a sestertius in some accounts.
Debasement of the silver coinage eroded the sestertius's purchasing power, and the antoninianus displaced it as the main small coin by the 260s and 270s. The state and counterfeiters withdrew sestertii to melt for the debased antoninianus, amplifying inflation. The coinage reforms of the fourth century did not revive the sestertius.
Hadrian issued a celebrated series of sestertii at Rome from roughly AD 130 to 133, pairing his portrait on the obverse with reverses that personified individual provinces or showed him addressing soldiers during imperial arrivals. The series includes one of the earliest coin representations of Britannia, an allegory later revived under Charles II for British coinage.