That is, the purchasing power of money is equal to the value of its own metal.
So ancient money was basically made of colored or precious metals such as copper, silver and gold.
At present, the purchasing power of money is guaranteed by the state, and its purchasing power has nothing to do with its own value
So now money is basically made of cheap materials such as paper (Australia also has some heterogeneous materials such as plastics). Even if some commemorative coins are metal coins under special circumstances, their face value far exceeds the value of their own metal.
Extended data:
The earliest currency in China was seashells. Shells are often found in prehistoric Yangshao, Longshan and Dawenkou cultural sites, Erlitou cultural sites in Xia Dynasty and tombs in Shang and Zhou Dynasties, and there is a record in "On Wrong Coins of Salt and Iron" that "Xia Hou took Xuanbei".
Shellfish is a distant product produced in the warm sea in the south, and it is a beautiful and precious ornament.
It can be traced back to the Xia Dynasty, when China entered the class society and the country came into being. Shang Dynasty and Western Zhou Dynasty were the main currencies in circulation. There were also imitation copper shells without words in the late Shang Dynasty and the Western Zhou Dynasty.
By the Eastern Zhou Dynasty, it developed into a bronze shell ant nose coin with inscriptions, forming a formal metal coin, which was mainly circulated in the southern Chu area.
The other earliest currencies were copper coins and knives. Money and knives are agricultural tools and multi-purpose tools. They are also transferable properties, which have been unearthed in the ruins of the Yin and Zhou Dynasties or earlier.
Probably in the late Shang Dynasty and the Western Zhou Dynasty, these bronze tools formed universal equivalents in different regions. Currency, _ developed into a full-time currency in the Spring and Autumn Period, and was called empty cloth by later generations;
Although the basic structure of bronze shovel is preserved, it has a slender spear, but it is not suitable for wooden handle and cannot be used as a tool, which has become a typical pre-Qin coin. In the Warring States period, it further developed into cloth coins with small shovel-shaped copper sheets, which were mainly distributed in the northern Zhou, Jin, Zheng and Wei regions. Bronze knives developed into knife coins in the Eastern Zhou Dynasty, mainly distributed in Qi, Yan and Zhao areas.
References:
Baidu Encyclopedia-China Ancient Currency