Obsidian: Volcanic Glass


Obsidian is a naturally occurring volcanic glass formed when molten lava from a volcano cools rapidly with minimal crystal growth. It is an igneous rock, meaning it forms from the cooling and solidification of magma or lava.

The rapid cooling of lava occurs when it comes into contact with water, air, or another cool surface. This rapid cooling prevents the atoms in the molten rock from arranging themselves into a crystalline structure, resulting in a glassy texture.

Obsidian: Definition, Properties & Uses
Obsidian: Volcanic Glass. Snowflake Obsidian

Properties of Obsidian

Obsidian is marked by its absence of crystals. To understand the formation of obsidian, we must first review how crystals form. You can think of the components in minerals as building blocks. In order for minerals to grow, the correct blocks must be present and they must be able to connect in the lava.

The chains of polymers in the felsic lava get in the way of mineral components connecting with each other to form crystals. Also, the overall high viscosity of the lava prevents much movement from occurring. Because crystals cannot form in this situation, the lava cools into a volcanic glass containing no crystals!

  • Composition: Obsidian is mainly composed of silica (SiO2) with smaller amounts of other elements like aluminum, iron, magnesium, sodium, and potassium.
  • Hardness: Obsidian is quite hard, with a Mohs hardness of 5 to 7. This makes it suitable for cutting and piercing tools.
  • Brittleness: While hard, obsidian is also brittle and easily breaks with sharp edges.
  • Amorphous: Unlike crystalline rocks, obsidian lacks a crystal structure and is considered a natural glass.
  • Color: Its most common color is black, but it can also appear in brown, gray, red, or even green.
  • Luster: Obsidian has a glassy or vitreous luster, reflecting light and creating a smooth, shiny appearance.
  • Fracture: It fractures with conchoidal fractures, meaning it breaks with smooth, curved surfaces resembling the inside of a shell.
  • Texture: Obsidian has a glassy texture, meaning it is smooth and non-porous. It can also be very sharp when broken, making it a valuable tool material.

Obsidian: Volcanic Glass
Oregon Rainbow Aurora Borealis Obsidian
Credit :OldGrowthForests

This breakdown of obsidian is accelerated by the presence of water. Having a low water content when newly formed, typically less than 1% water by weight, obsidian becomes progressively hydrated when exposed to groundwater, forming perlite.

Obsidian Appearance

Pure obsidian is usually dark in appearance, though the color varies depending on the presence of impurities. Iron and magnesium typically give the obsidian a dark brown to black color. Very few samples are nearly colorless. The color of obsidian depends on the chemical composition of the impurities.

In some stones, the inclusion of small, white, radially clustered crystals of cristobalite in the black glass produce a blotchy or snowflake pattern (snowflake obsidian). 

Obsidian may contain patterns of gas bubbles remaining from the lava flow, aligned along layers created as the molten rock was flowing before being cooled. These bubbles can produce interesting effects such as a golden sheen (sheen obsidian).

An iridescent, rainbow-like sheen (rainbow obsidian) is caused by inclusions of magnetite nanoparticles. California Rainbow Obsidian

Green Obsidian: Occurs due to the presence of ferrous iron or small inclusions of other minerals like olivine.

Obsidian: Definition, Properties & Uses


Obsidian can be found in locations which have experienced rhyolitic eruptions. It can be found in Argentina, Armenia, Azerbaijan, Australia, Canada, Chile, Georgia, Greece, El Salvador, Guatemala, Iceland, Italy, Japan, Kenya, Mexico, New Zealand, Papua New Guinea, Peru, Scotland, Turkey and the United States.

Obsidian flows which may be hiked on are found within the calderas of Newberry Volcano and Medicine Lake Volcano in the Cascade Range of western North America, and at Inyo Craters east of the Sierra Nevada in California.

Yellowstone National Park has a mountainside containing obsidian located between Mammoth Hot Springs and the Norris Geyser Basin, and deposits can be found in many other western U.S. states including Arizona, Colorado, New Mexico, Texas, Utah, Washington, Oregon and Idaho. Obsidian can also be found in the eastern U.S. states of Virginia, as well as Pennsylvania and North Carolina.

Uses of Obsidian

One of the most familiar associations with obsidian is its use in arrowheads by Native Americans. Because the natural glass does not have an orderly internal structure, obsidian will break into conchoidal fracture. Conchoidal fracture is a smooth scallop-shaped surface formed when a glassy material like obsidian is broken. The tendency of obsidian to form conchoidal fracture is what allows it to form such sharp surfaces. People learned to skillfully chip away and sculpt obsidian to form extremely sharp and effective cutting tools.

Jewelry: Obsidian is a popular gemstone due to its unique black color and luster. It is often used to make beads, cabochons, and tumbled stones.

Cutting tools: Obsidian can be fractured to create extremely sharp edges, making it ideal for tools such as knives, scrapers, and arrowheads. In fact, obsidian blades can be as sharp as surgical steel scalpels.

Mirrors: In the past, obsidian was used to create reflective surfaces, particularly in Mesoamerican cultures.

Construction: Crushed obsidian can be used as an aggregate in concrete and other building materials.

Electronics: Obsidian can be used in the production of certain electronic components, such as transistors.

Historical Uses of Obsidian:

Weapons: For centuries, obsidian was used to make weapons such as swords, axes, and spears. Its sharpness and durability made it a valuable tool for warfare and hunting.

Ritual objects: Obsidian was also used to create ritual objects, such as masks and figurines, in various cultures around the world.

Trading: Obsidian was a valuable trade commodity in ancient times, and its presence in archaeological sites can provide clues about trade networks and cultural interactions.

Metaphysical Uses of Obsidian

Healing: Some people believe that obsidian possesses certain healing properties. It is often used to promote emotional well-being, clear negativity, and dispel confusion.

Meditation: Obsidian is also used in meditation practices to help focus the mind and achieve spiritual clarity.

Grounding: Due to its dark color and association with the Earth element, obsidian is believed to have grounding properties, helping individuals feel more connected to the physical world.

Overall, obsidian is a versatile material with a rich history and diverse range of uses. It continues to be valued for its beauty, functionality, and cultural significance.

See also:
Top 7 Differences Between Metamorphic Rocks and Igneous Rocks 
California Rainbow Obsidian Is a Natural Wonder (Photos)
The Texture of Igneous Rocks
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