A field like the Roman coinage is immense, for its duration (one millenium) as well as for its geographical extent (all the Mediterranean area) and for the variety of its production. Many collectors however make the best of it, while limiting themselves to a particular aspect such as the portraits of emperors – which are often of great quality - the production of one city, the mythological characters on coins, etc.
It is worth mentioning that many coins are affordable for little money, and - for treasure hunters - that it is not at all unusual to find some in the French countryside, with the satisfaction of saying “I found it by myself” (but keep in mind that the use of the metal detectors is generally prohibited !). Lastly, since this coinage has been thoroughly studied, you may rely on precise guides to improve your knowledge on this field.

Overview

The Roman coinage extends over a whole millenium (from the 4th century B.C to the 5th century A.D.) and on all the Mediterranean area. It however appeared late, in comparison with other countries of this area.
The coinage of the Roman Republic began with some silver currencies allowing to trade with the Greek colonies of southern Italy, and also especially with large cast bronze coins in central Italy where this resource was more common.
At the beginning of the Roman Empire, under Augustus, appeared also gold coinage. The political and economic crises of the 3rd century resulted in an inflation which lead to a collapse of the currencies’ value. In the 4th century, Diocletian tried in vain to revalue the silver and bronze currencies, but Constantin Ist managed to create a stable system based on the solidus of 4,5 grams gold, which will kept its stability from the Eastern Roman empire to the Byzantine empire, where it was called nomisma. It remained unaffected by devaluation until the 11th century, under the Comneni, an extraordinary seven centuries stability. .

The coinage of the Roman Republic

Origin
Until 400 B.C., the Romans practised barter often based on the “heads of cattle”, though knowing the Greek currencies. They then started to build up a system based on bronze ingots, the Aes rude (rough ore), which had neither a fixed weight nor a precise shape, and none of the marks which define a currency. The first markings represented cattle - what remembers the initial barter - or weapons. At the beginning of the 3rd century, the bronze masses took a more regular shape; if their weight remains unfixed, it is at least engraved on them (Aes signatum).

The Aes grave
At last, in 289 B.C., the Aes grave (heavy ore) appeared, a real currency having the shape of a bronze disc, weighing a pound (324 grams), which was divided into 2 semis, or 3 triens, or 4 quadrans, or 6 sextans, or 12 uncias (ounces). One may notice here the interest of this “base 12” counting to define fractions, compared to decimals! The coins were cast, and showed a relief depending on their value: Janus for the As, Minerva for the Semis, etc. About 280-270, the coins began to be struck. When the Punic Wars began, they required considerable spending for building the fleet: in order to cope with this, Rome devaluated its currency, that is to say reduced the bronze weight of each coin. Towards 250, the as weighed only ca. 280 grams.
Multiples of them appeared, like the sesterce and the dupondius. Silver, which was used only in imitation of the Greek colonies of southern Italy during their conquest, entered in 214 B.C. in the Roman coinage, partly thanks to the silver that had been taken during the plundering of Syracuse in 212. The Denarius, which weighs 4,5 grams silver, was worth 10 as… which had been reduced in the meanwhile to 53 grams bronze! On Denarii, the allegorical characters inspired by mythology dominate (for example, a victory on a chariot dragged by two horses).

Devaluations went on, sometimes affecting silver, sometimes bronze currency. The sesterce (1/4 denarius) became the current unit of the 2nd century. Ca. 140 B.C., it is worth 4 as. Inscriptions appear on the background of the coin, as well as the names of the monetary magistrates. The allegorical images from mythology are reused for their glory.
In the 1st century B.C., it was disputed for a long time whether the actors of civil wars should engrave their portrait on coins. About 80 B.C., the production of bronze coins almost stopped, but the as reappeared sometimes during the later periods.

The coinage of the early Roman empire

Augustus, who became emperor in 23 A.D., reorganized the monetary system and introduced a gold currency, the aureus of 7,85 grams, which was worth 25 denarii (or 100 sesterces, or 200 dupondii). The sesterce - from then on made of brass - remained the usual currency, and all the successive emperors were watchful to have their portraits on them, and even those of the members of their family. The inscriptions on the coins (legend) deliver the whole imperial titulature. The reverse of the coins is a space for propaganda, using largely allegories.
The silver Cistophore was used in Asia Minor, where its worth was 3 Roman denarii or 4 Greek drachms.

In the course of years, the Denarius was devaluated in turn:
- under Severe Septime, ca. 200 A.D., its silver content passed from 70% to 50% silver, and the weight of the aureus from 8,1 to 6,5 grams gold,
- in 215, Caracalla introduced a double-denarius called antoninian which weighed only 1,5 denarius (5,07g), and contained 50% silver,
- in 258, under the reign of Valerian, the antoninian silver content was approximately 20%,
- under the reign of Claude the Gothic (268-270), its content fell to only 3 or 4% and its aspect was closer to that of a bronze coin.

The sesterce was produced up to the reign of Gallienus (260-268). In 274, Aurelian reformed the currency system and increased the weight of the antoninian, which was then called aurelianus. He also attempted to reintroduce the bronze as and the sesterce.

The coinage of the late Roman empire

Diocletian, in 294, attempted a new monetary reform.
The aureus passed from 1/50 to 1/60 of gold pound. He created also the argenteus (3,4g silver), the follis or nummus made of silver plated bronze, which is worth 5 denarii, and the billon which is worth 2 denarii.
Since inflation persists, Constantine (306-337) created finally a new system, where the solidus (4,5g gold) replaced the aureus. It was supposed to ensure – as the name shows - a certain stability – and it reached it. Its submultiples were the semissis (1/2), the tremissis (1/3), the miliares, the silicus, the half silicus.
In the 5th century, the monetary system changed. Its most important components were the solidus and the tremissis. The production of silver coins stopped practically. As for bronze coins, they survived after 415 in the Western and 425 in the Eastern Roman empire as a small coin weighing around one gram.