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People began using glass long before markets even existed. Early humans used obsidian—molten lava that cooled quickly—to make simple cutting tools and arrowheads. And although evidence from beads and other archaeological finds indicates that people figured out by 4,000 B.C.E. how to form glass coatings (glazes) by melting silica, the main component of sand, it would be another 2,500 years until ancient Mesopotamians got the hang of making hollow glass vessels, which they used for storing oils.