Obsidian
Volcanic glass used for sharp tools since prehistory.
Obsidian is a naturally occurring volcanic glass formed when felsic lava cools rapidly with minimal crystal growth. It is an igneous rock rich in lighter elements such as silicon, oxygen, aluminium, sodium, and potassium, commonly found within the margins of rhyolitic lava flows. Obsidian is hard, brittle, and amorphous, fracturing with very sharp edges, which made it valuable for manufacturing cutting and piercing tools in the past and for experimental use as surgical scalpel blades.
- type
- Volcanic glass (mineraloid)
- composition
- Mainly SiO₂ (70% or more by weight) with variable oxides of aluminium, iron, potassium, sodium, and calcium
- color
- Usually dark, often black; can be nearly colorless, or show snowflake, sheen, fire, or rainbow patterns
- hardness
- Hard and brittle
- formation
- Rapid cooling of felsic lava, extrusively or intrusively
- age_limit
- Rare older than Miocene due to devitrification
- major_sources
- Argentina, Armenia, Azerbaijan, Australia, Canada, Chile, Georgia, Ecuador, El Salvador, Greece, Guatemala, Hungary, Iceland, Indonesia, Italy, Japan, Kenya, Mexico, New Zealand, Papua New Guinea, Per
Lore & Background
Obsidian is formed from quickly cooled lava, either extrusively at the edges of felsic lava flows or volcanic domes, or when lava cools during sudden contact with water or air. Intrusive formation can occur along the edges of a dike. The high silica content gives the lava high viscosity, which inhibits diffusion of atoms and nucleation of crystals, resulting in natural glass. Obsidian is metastable at Earth's surface; over time it devitrifies into fine-grained mineral crystals, accelerated by water, so obsidian older than Miocene is rare. Exceptionally old examples include a Cretaceous welded tuff and a partially devitrified Ordovician perlite.
Reader's Guide
Obsidian's significance lies in its use by prehistoric and historic cultures for cutting tools, weapons, and mirrors due to its ability to fracture into extremely sharp edges. Archaeological evidence of usage dates to the Acheulian age (beginning 1.5 million years BP), with bladelet manufacture reaching sophistication by the late Neolithic. Obsidian was traded over long distances, as seen in artifacts from the Levant, Mesopotamia, Europe, and the Americas. Modern archaeologists use obsidian hydration dating and techniques like X-ray fluorescence and neutron activation analysis to trace artifact origins and ancient trade routes. The material is also notable for its variety of colors and patterns caused by impurities and inclusions, such as snowflake, sheen, fire, and rainbow obsidian.
Did You Know?
- Obsidian is not a true mineral because it is not crystalline and its composition is too variable; it is sometimes classified as a mineraloid.
- The Roman writer Pliny the Elder mentioned obsidian in his Natural History, naming it after a Roman explorer called Obsidius who discovered it in Ethiopia.
- Obsidian bladelets were used in ritual circumcisions and cutting of umbilical cords of newborns.
Common Misconceptions (Editorial)
- Some people think obsidian is a true mineral, but it is not crystalline and has a variable composition, making it a mineraloid rather than a mineral. - Another common misconception is that obsidian can last indefinitely; in fact, it is metastable and devitrifies over time, so obsidian older than the Miocene epoch is rare.
Why It Matters (Editorial)
Obsidian endures in human significance because its sharp, brittle fractures once made it indispensable for tools and weaponry, linking prehistoric innovation to modern experimental surgery. Its glassy, often black beauty also continues to captivate artists and collectors, while its scientific value as a record of rapid volcanic cooling offers a window into Earth's fiery geology.
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