Volcanic glass carries a chemical fingerprint that maps ancient trade routes
Each volcano, and sometimes each single eruption, yields obsidian with its own chemical signature. That lets archaeologists match a blade to the hillside it came from. Pieces from the Yellowstone region have turned up at Hopewell sites in Ohio, more than 1,500 miles away, evidence of exchange networks nobody wrote down.
Obsidian forms when silica-rich lava cools too fast for crystals to grow. That lava is so viscous that atoms struggle to move through it, blocking the first step of crystal formation, so the melt freezes into glass. Usually 70 percent or more of it by weight is silicon dioxide. Because it has no crystal structure and a variable makeup, it is not strictly a mineral. It is also unstable over geological time, slowly turning into fine crystals, so obsidian older than the Miocene is rare. Its colours come from impurities: magnetite nanoparticles can give a rainbow sheen, and white clusters of cristobalite make the blotchy variety called snowflake obsidian.
Being brittle and amorphous, it breaks with extremely sharp edges, which made it prized for knapping into blades and arrowheads. The earliest known use comes from Kariandusi in Kenya, dated to 700,000 BC. Anatolian obsidian reached the Levant from around 12,500 BC, a Bronze Age site in Turkey has yielded scalpels of it predating 2100 BC, and experimental surgical blades have been made from it in modern times.
Tracing techniques such as X-ray fluorescence have revealed remarkable reach. Greek pieces can be pinned to Milos, Nisyros or Gyali, and eight Chalcolithic artifacts at Gilat in Israel were traced to Anatolia. In Chile, tools made from Chaitén Volcano obsidian travelled 400 km in both directions. The Lapita culture moved it across the Pacific around 1000 BC.
In Mesoamerica it lined the wooden macuahuitl sword, and Teotihuacan may have monopolised the prized green Pachuca deposit. Aztec priests used obsidian mirrors for visions, linked to Tezcatlipoca, whose Nahuatl name means Smoking Mirror. John Dee later owned one brought from Mexico to Europe between 1527 and 1530.
Source: Obsidian
