The aureus was a gold coin

Gold coin

A gold coin is a coin made mostly or entirely of gold. Gold has been used for coins practically since the invention of coinage, originally because of gold's intrinsic value...

 of ancient Rome

Ancient Rome

Ancient Rome was a thriving civilization that grew on the Italian Peninsula as early as the 8th century BC. Located along the Mediterranean Sea and centered on the city of Rome, it expanded to one of the largest empires in the ancient world....

 valued at 25 silver denarii

Denarius

In the Roman currency system, the denarius 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 aureus was regularly issued from the 1st century BC to the beginning of the 4th century, when it was replaced by the solidus

Solidus (coin)

The solidus was originally a gold coin issued by the Romans, and a weight measure for gold more generally, corresponding to 4.5 grams.-Roman and Byzantine coinage:...

. The aureus is about the same size as the denarius, but is heavier due to the higher density

Density

The mass density or density of a material is defined as its mass per unit volume. The symbol most often used for density is ρ . In some cases , density is also defined as its weight per unit volume; although, this quantity is more properly called specific weight...

 of gold

Gold

Gold is a chemical element with the symbol Au and an atomic number of 79. Gold is a dense, soft, shiny, malleable and ductile metal. Pure gold has a bright yellow color and luster traditionally considered attractive, which it maintains without oxidizing in air or water. Chemically, gold is a...

.

Before the time of Julius Caesar

Julius Caesar

Gaius Julius Caesar was a Roman general and statesman and a distinguished writer of Latin prose. He played a critical role in the gradual transformation of the Roman Republic into the Roman Empire....

 the aureus was struck very infrequently, usually to make large payments from captured booty.