Původní text - Anglicky - Naturally occurring glass

Naturally occurring glass, especially obsidian, has been used by many Stone Age societies across the globe for the production of sharp cutting tools and, due to its limited source areas, was extensively traded. According to Pliny the Elder, Phoenician traders were the first to stumble upon glass manufacturing techniques at the site of the Belus River. Agricola, De re metallica, reported a traditional serendipitous "discovery" tale of familiar type