What does it mean that metal money was used more in the Spring and Autumn Period?

Shell coins have always been regarded as an important currency form in China's slave society, and most countries in the world basically used shells as circulation currency at that time. The reason why seashells are used as money is mainly because seashells are commodities and have use value. In ancient times, people generally used shells as decorations, so they have great use value and necessary conditions for circulation. At the same time, the shell is firm, not easy to be damaged and easy to carry. The most important thing is that seashells are suitable as units of measurement, so they are used as money in summer. Of course, besides seashells, there were other physical currencies in Xia, Shang and Zhou Dynasties, mainly including tortoise shells, which were also used as money because of their use value. Especially after Shang Dynasty, tortoise shells were not only used as money, but also used for divination, but they began to withdraw from the circulation field in Shang Dynasty and completely withdrew from the monetary system in Qin Dynasty; Zhuyu. Although jade beads are used as currency, they are widely used in life in a short time because of their high value and easy wearing, and their circulation scope is limited to the aristocratic ruling class. Tools. Tools were mainly used as money in Shang Dynasty and Western Zhou Dynasty, including axes, copper shovels and knives.