One of the coin collector's most coveted treasures is an Ancient Roman denarius minted around 45 BCE, featuring a laurel-wreathed profile of the statesman Julius Caesar. The four-gram coin contains more than 90% silver, but precious metals aren't what make the denarii valuable. Nor is it the rarity of mint condition examples that drives the value of the coin to upwards of $1,000. Rather, it's the historical significance: The denarius is the first coin that commemorated a living Roman leader, minted as the republic slid into empire.