Oregon Rainbow Aurora Borealis Obsidian
Credit :OldGrowthForests

Obsidian Definition and Origin

If you've ever held a black glassy arrowhead, you've held obsidian. Before it was expertly shaped into a sharp point, obsidian was formed through a complex and rare earth process.
Obsidian is a naturally occurring volcanic glass. It is formed during the eruption of felsic lavas, which are distinguished by having high concentrations of the chemical element silica. Because of their high silica content, felsic lavas do not behave like the mafic, or silica-poor, lavas we see on the island of Hawaii. Silica forms bonds with oxygen in lava creating linked molecule chains. These linked molecule chains are called polymers and the process of forming them is called polymerization. Polymers increase the viscosity, or resistance to flow, of the lava. A more familiar example of substances with differing viscosities is the difference between honey and water. Water has very low viscosity, so it flows very easily, whereas honey has a high viscosity and flows much more slowly. The silica-rich lavas that form obsidian flow extremely slowly due to the effects of polymerization of the silica atoms.


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!

Close up of glassy obsidian at Obsidian Cliff; RG Johnsson; 1964; Volcanics

Obsidian is mineral-like, but not a true mineral because as a glass it is not crystalline; in addition, its composition is too complex to comprise a single mineral. It is sometimes classified as a mineraloid. Though obsidian is usually dark in color similar to mafic rocks such as basalt, obsidian's composition is extremely felsic. Obsidian consists mainly of SiO2 (silicon dioxide), usually 70% or more. Crystalline rocks with obsidian's composition include granite and rhyolite. Because obsidian is metastable at the Earth's surface (over time the glass becomes fine-grained mineral crystals), no obsidian has been found that is older than Cretaceous age. 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.
 Snowflake Obsidian

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. 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.


A common misconception about obsidian is that its dark color is due to having crystallized from mafic lava. Mafic lavas crystallize to form basalt, which is also dark in color. Obsidian is commonly a translucent dark brown or black. Unlike basalt, obsidian's dark color is due to high amounts of impurities rather than the presence of dark colored minerals. The color of obsidian depends on the chemical composition of the impurities.

A unique type of obsidian is called snowflake obsidian due to the presence of radiating cristobalite, a light colored material. This obsidian variant is commonly used for jewelry.

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.

Though not approved by the US Food and Drug Administration (FDA) for use on humans, obsidian is used by some surgeons for scalpel blades, as well-crafted obsidian blades have a cutting edge many times sharper than high-quality steel surgical scalpels, the cutting edge of the blade being only about 3 nanometers thick.

Obsidian is also used for ornamental purposes and as a gemstone. It presents a different appearance depending on how it is cut: in one direction it is jet black, while in another it is glistening gray. "Apache tears" are small rounded obsidian nuggets often embedded within a grayish-white perlite matrix.

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