The History of Roman Coins - The Denarius
In the Roman currency system, the denarius (plural: denarii) was a small silver coin first minted in 211 BC. It was the most common coin produced for circulation but was slowly debased until its replacement by the antoninianus.
The denarius was first struck in or about 211 BC during the Roman Republic and at the same time as the Second Punic War, with a weight of 4.5 grams on average at the time or 1/72 of a Roman Pound. It remained at this weight for a while and then decreased to about 3.9 grams during the second century BC (a theoretical weight of 1/84 of a Roman pound). It then remained at almost this weight until the time of Nero, when it was reduced to 1/96 of a pound, or 3.4 grams. Debasement of the silver began under Nero. Later Roman emperors reduced it to a weight of 3 grams around the late 3rd century . The value at its introduction was 10 asses, giving the denarius its name which translates to 'containing ten'. In about 141 BC it was re-tariffed at 16 asses, to reflect the decrease in weight of the as. The denarius continued to be the main coin of the empire until it was replaced by the antoninianus in the middle of the 3rd century. The last issuance for this coin seems to be bronze coins issued by Aurelian between 270 and 275 AD, and in the first years of the reign of Diocletian. For more details, see the article 'Denarius' in A Dictionary of Ancient Roman Coins by John R. Melville-Jones (1990).  
Comparisons and silver content
It is problematic to give even rough comparative values for money from before the 20th century, due to vastly different types of products and of the impossibility of making an accurate price index based on vastly different spending proportions. Its purchasing power in terms of bread has been estimated at US$21, from 2005, in the first century. Classical historians regularly say that in the late Roman Republic and early Roman Empire the daily wage for an unskilled laborer and common soldier was 1 denarius without tax, or about US$20 in bread. (By comparison, a laborer earning the minimum wage in the United States makes US$58 for an 8-hour day, before taxes.) The actual silver content of the Denarius was about 50 grains, or 1/10 troy ounce under the Empire.
The fineness of the silver content varied with political and economic circumstances. By the reign of Gallienus, the Antoninianus was a copper coin with a thin silver wash. 
Even after the denarius was no longer regularly issued, it continued to be used as an accounting device and the name was applied to later Roman coins in a way that is not understood. The Arabs who conquered large parts of the Roman Empire issued their own Gold Dinar, from which the name Dinar of various present-day Arab currencies is derived. The lasting legacy of the denarius can be seen in the use of 'd' as the abbreviation for the British penny prior to 1971. It survived in France as the name of a coin, the denier. The denarius also survives in the common Arabic name for a currency unit, the dinar used from pre-Islamic times, and still used in several modern Arabic-speaking nations. Currency unit in former Yugoslavia and nowadays in Serbia is dinar which also has its origins in the Latin word denarius. The Macedonian currency denar is also derived from the Roman denarius. The Italian word denaro, Spanish word dinero, the Portuguese word dinheiro, the Slovene word denar and the Catalan word diner, all meaning money, are also derived from Latin 'denarius'.
The gold aureus seems to have been a 'currency of account', a denomination not commonly seen in daily transactions due to its high value. Numismatists think that the aureus was used to pay bonuses to the legions at the accession of new emperors. It was valued at 25 denarii.
1 gold aureus = 2 gold quinarii = 25 silver denarii = 50 silver quinarii =100 bronze sestertii = 200 bronze dupondii = 400 copper as = 800 copper semisses = 1600 copper quadrans
The Bible refers to the denarius as a day's wage for a common laborer (Matthew 20:2 ; John 12:5 ).The value of the denarius is referred to, though perhaps not literally, in the Bible at Revelation 6:6: 'And I heard something like a voice in the center of the four living creatures saying, 'A quart of wheat for a denarius, and three quarts of barley for a denarius [Latin Vulgate: bilibris tritici denario et tres bilibres hordei denario]; and do not damage the oil and the wine.''
- The History of Roman Coins
- The History of Roman Coins - The Aureus
- The History of Roman Coins - The Denarius
- The History of Roman Coins - The Sestertius
- The History of Roman Coins - The Antoninianus