Obsidian
Volcanic glass used for sharp tools since prehistory.
Obsidian is a naturally occurring volcanic glass, an igneous rock formed when felsic lava cools so rapidly that mineral crystals have little chance to grow. This lava, rich in lighter elements like silicon, oxygen, aluminum, sodium, and potassium, is typically found at the edges of rhyolitic lava flows, known as obsidian flows. The high silica content of these flows makes them extremely viscous, which hinders the movement of atoms and prevents the nucleation necessary for crystal formation. Combined with rapid cooling, this process results in a natural glass. Obsidian is hard, brittle, and amorphous, causing it to fracture with exceptionally sharp edges. This property made it valuable for creating cutting and piercing tools in the past, and it has been experimentally used for surgical scalpel blades.
The Roman writer Pliny the Elder, in his work *Natural History*, mentions a volcanic glass called obsidian, reportedly discovered in Ethiopia by a Roman explorer named Obsidius. Obsidian forms extrusively when felsic lava cools quickly at the edges of a flow or volcanic dome, or upon sudden contact with water or air. It can also form intrusively along the edges of a dike. Though often dark in color like mafic rocks, obsidian is extremely felsic, consisting mainly of silicon dioxide (typically 70% or more by weight), with variable amounts of other oxides. Because it is a glass and not crystalline, and its composition is too variable, it is not a true mineral but is sometimes classified as a mineraloid. Obsidian is metastable at the Earth's surface and will devitrify over time into fine-grained crystals, a process accelerated by water. Consequently, obsidian older than the Miocene epoch is rare, though exceptions include a Cretaceous welded tuff and a partially devitrified Ordovician perlite. Pure obsidian is usually dark, with black varieties often containing nanoinclusions of magnetite. Other colors and patterns arise from impurities: snowflake obsidian contains white cristobalite crystals; sheen obsidian has aligned gas bubbles; fire obsidian displays iridescence from magnetite nanoparticles; and rainbow obsidian from Mexico shows striped colors due to hedenbergite nanorods. Obsidian is found near volcanoes with rhyolitic eruptions across many countries, including the United States, Mexico, Japan, Italy, and Turkey, with notable large flows in the Cascade Rang
- 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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