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What is the black stone that is sharp?

The black stone referenced in the title likely refers to obsidian, a naturally occurring volcanic glass. Obsidian is formed when lava from volcanic eruptions cools rapidly with minimal crystal growth. This allows the lava to solidify into a glass-like structure. Obsidian can form very sharp edges, making it useful for tools and weapons in ancient cultures. This article will provide an overview of obsidian, including its physical properties, how it forms, where it can be found, its use throughout human history, and other key facts about this sharp black stone.

What is Obsidian?

Obsidian is a naturally occurring volcanic glass formed from rapidly cooled lava. Its key characteristics include:

  • Composition: 70-80% silicon dioxide (SiO2), similar to window glass
  • Amorphous structure: Obsidian has no crystalline structure due to its rapid formation, giving it a smooth, glass-like texture
  • Color: Typically black or very dark gray, though can also be brown, tan, or greenish depending on mineral impurities
  • Luster: Vitreous, appearing shiny or glassy
  • Hardness: 5-5.5 on the Mohs hardness scale
  • Fracture: Conchoidal – fractures and breaks in curved shapes with very sharp edges
  • Density: 2.35-2.60 g/cm3
  • Melting point: 1100-1150°C

Some key things to note are obsidian’s amorphous structure and conchoidal fracture which allow it to break into pieces with incredibly sharp edges. This made obsidian a invaluable resource for crafting extremely sharp tools and weapons in ancient societies across the world.

How Does Obsidian Form?

Obsidian is formed from lava that cools very rapidly, before crystallization can occur. This requires specific volcanic conditions:

  • Rhyolitic lava composition – high in silica content (typically 72% SiO2 or higher)
  • Fast cooling – lava cools at a rate of 1000°C/day or faster
  • Gaseous eruptions – allow lava to spread thinly with maximum surface exposure

When these conditions are met, the rhyolitic lava can immediately solidify into obsidian glass instead of having time to form crystals. Cooling can occur on exposed lava surfaces or within the lava flow due to embedded air pockets. Common obsidian formation environments include:

  • Fissure eruptions – lava emerging from cracks in the ground promotes rapid cooling
  • Pyroclastic flows – hot ash and rock fragments cool while airborne before settling
  • Domes/Flow edges – edges of domes or lava flows are directly exposed to air/water

Obsidian’s natural glass properties are a direct result of this unusually fast cooling during formation.

Where is Obsidian Found?

As a volcanic rock, obsidian can be found anywhere rhyolitic lava flows under the right cooling conditions, typically near convergent plate boundaries where subduction zones generate volcanism. Major global obsidian sources include:

  • North America – Western US through Mexico and Central America
  • South America – Andes mountains from Chile to Peru
  • Mediterranean – Greece, Italy, Turkey, Armenia
  • East Africa – Kenya, Tanzania, Ethiopia
  • Polynesia/Micronesia – New Zealand, Papua New Guinea, Japan

Notable obsidian locations include Obsidian Cliff in Yellowstone National Park, North America and the Easter Island deposits in the southeastern Pacific Ocean. Archaeological evidence indicates obsidian trade routes spread this resource far from its geologic sources amongst ancient cultures who viewed it as a highly valued commodity.

Major Obsidian Sources and Trade Routes in North and South America

Obsidian Source Location Trade Reach
Valles Caldera New Mexico American southwest
Casa Diablo California Western North America
Apache Tears Arizona Southwestern United States
Ixtepeque Guatemala Mesoamerica
Chivay Peru Central Andes

Obsidian Use Throughout History

Obsidian played an integral role in toolmaking, weaponry, and rituals across many ancient civilizations. Some major uses include:

Cutting Tools

Sharp obsidian flakes were used as knives, razors, and surgical blades as early as the Upper Paleolithic age. Ancient people could fashion precise tools from obsidian through lithic reduction techniques like percussion flaking and pressure flaking. Obsidian blades can have edges just nanometers thick, making them far superior to metals for cutting tasks.

Projectile Points

The earliest identified obsidian artifacts are Paleolithic projectile points from Ethiopia dating to ~700,000 BC. Obsidian’s ability to fracture into extremely sharp edges made it an ideal material for arrowheads, spears, and darts through the ages. These deadly projectiles continued to be used against Spanish conquistadors in Central America as late as the 16th century.

Rituals and Art

Many cultures incorporated obsidian into meaningful rituals, ceremonies, and statues. For example, the Mayans viewed it as a sacred material connected with deities, sacrifice, and shamanic scrying. Obsidian mirrors were widely used in Mesoamerican divinatory rituals. The Inca civilization in South America also used obsidian in ornate jewelry, decorations, and tools reserved for elites.


Although limited geographically, major obsidian sources were heavily traded and distributed across large regions in antiquity. Famed obsidian routes include the Mediterranean trade networks, rituals and art

Other Key Facts About Obsidian

In addition to its important role in human history, obsidian has many other interesting properties and uses in modern times. Here are some key facts:

  • Natural glass composition makes obsidian highly reflective and refractive
  • Alters passing light due to lack of crystalline structure, producing an iridescent sheen
  • In some forms presents banding patterns due to flow dynamics
  • Extremely high water content when first formed, but can become progressively hydrated/weathered over time
  • Measuring water content allows obsidian hydration dating of artifacts
  • Magnetic properties including strong remnant magnetization provide additional dating methods
  • Pure obsidian is usually dark but impurities can produce green, brown, red or orange colorations
  • Has been used for making glass sculptures, mirrors and decorative objects
  • Currently used surgically for scalpels and other cutting applications

Obsidian is truly a unique and fascinating geologic material that has been one of humanity’s most valued resources since its first uses by ancient hominins. Its distinct natural properties produced an invaluable toolstone and object of cultural significance across countless civilizations. Obsidian continues to be studied and utilized today for both geologic research and practical applications. Whether occurring naturally or shaped into a functional object, obsidian is unmistakable in appearance – the glossy, black volcanic glass that fractures into the sharpest edges.


In summary, obsidian is a naturally formed volcanic glass with a unique amorphous structure that allows it to break into extremely sharp edges. It is produced globally in areas of rhyolitic lava flows, with major deposits found in North and South America, the Mediterranean, and East Africa among other locations. Obsidian played an integral role in ancient tools and weaponry dating back to the earliest humans. Its ability to fracture into versatile sharp pieces made obsidian a highly traded and culturally significant resource for cutting, projectiles, decorative art, and rituals across many civilizations. In modern times, obsidian is still valued for specialized surgical blades and geologic research. This black volcanic glass remains one of the most distinctive and useful rocks found in nature.