We can count on it to not dissolve, mildew, or catch fire. Gold was, and still is, pretty scarce. In all of history, the total amount of gold ever mined totals about 165,000 tons, a weight equal to that of one and a half US aircraft carriers. Coins made from silver (the word "coin" means "invent") have many of the same attributes as their gold cousins and have been in use for centuries.
While silver and gold have many positive characteristics, they also have a key drawback: They're heavy.
In the 13th century, the Chinese emperor Kublai Khan changed the way we think about money. Khan's great insight was that money—in the form of sea shells or gold coins—was valuable only if people believed in it. He also knew that the different regions of China were issuing their own coins, which made trade within his empire more difficult.