The metal age was known as the Chalcolithic age because it was during this time period that human societies began to experiment with metals such as copper and reorganize their society due to advancement of technology.
For further reading check the following articles:
|NCERT notes on Rigveda||Vedic literature|
|100 Must-Know Facts about Indus Valley Civilization (IVC)||UPSC FAQ|
|UPSC Syllabus||Sangam Literature|