A Look at the Multitude of Variations of Obsidian Rock

Close up of native american obsidian arrowhead paiute indian stone tool in dirt from Oregon great basin desert
Obsidian arrowheads were common. Tyler Hulett / Getty Images
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Obsidian Flow

From a distance
Obsidian Picture Gallery. Photo courtesy bdsworld of Flickr.com under Creative Commons license

Obsidian is an extreme variety of igneous rock with a glassy texture. Most popular accounts say that obsidian forms when lava cools very quickly, but that is not quite accurate. Obsidian starts with a lava very high in silica (more than about 70 percent), such as a rhyolite. The many strong chemical bonds between silicon and oxygen make such a lava very viscous, but equally important is that the temperature range between fully liquid and fully solid is very small. Thus obsidian does not need to cool especially fast because it solidifies especially quickly. Another factor is that a low water content may inhibit crystallization. View pictures of obsidian in this gallery.

Big Obsidian Flow, in Newberry Caldera in central Oregon, displays the rugged surface of the highly viscous lava that forms obsidian.

Learn More About Igneous Rocks

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Obsidian Blocks

Typical of flows
Obsidian Picture Gallery. Photo courtesy yananine of Flickr.com under Creative Commons license

Obsidian flows develop a blocky surface as their outer shell quickly solidifies. This is from the Big Obsidian Flow in Newberry Caldera, Oregon.

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Obsidian Flow Texture

Complex textures
Obsidian Picture Gallery. Photo (c) 2008 Andrew Alden, licensed to About.com ( fair use policy)

Obsidian may display complex folding and segregation of minerals in bands and round masses consisting of feldspar or cristobalite (high-temperature quartz).

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Spherulites in Obsidian

Little mineral clusters
Obsidian Picture Gallery. Photo (c) 2008 Andrew Alden, licensed to About.com ( fair use policy)

Obsidian flows may contain droplets of fine-grained feldspar or quartz. These are not amygdules as they were never empty; instead, they are called spherulites.

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Fresh Obsidian

Black as anthracite
Obsidian Picture Gallery. Photo (c) 2007 Andrew Alden, licensed to About.com ( fair use policy)

This obsidian hand specimen was mined from Red Island lava dome near the Salton Sea in southern California. Usually black, obsidian can also be red or gray, streaked and mottled, and even clear.

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Obsidian Cobble

Just slightly weathered
Obsidian Picture Gallery. Photo (c) 2007 Andrew Alden, licensed to About.com ( fair use policy)

The shell-shaped conchoidal fracture on this obsidian cobble is typical of glassy rocks like obsidian or microcrystalline rocks like chert.

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Obsidian Hydration Rind

Frosty coating
Obsidian Picture Gallery. Photo (c) 2007 Andrew Alden, licensed to About.com ( fair use policy)

Obsidian combines with water and begins to break down into a frosty coating. Internal water can convert the whole rock into perlite

This obsidian lump comes from California's Napa Valley, where volcanic deposits help create the rich soil there. The outer rind shows signs of hydration from being buried in the soil for thousands of years. The thickness of this hydration rind is used to show the age of obsidian, and hence the eruption that produced it.

Note the faint bands on the outer surface. They result from mixing of the thick magma underground. The clean, black fractured surface shows why Obsidian was valued by the native people for making arrowheads and other tools. Chunks of obsidian are found far from their place of origin because of prehistoric trading, and therefore they bear cultural as well as geologic information.

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Weathering of Obsidian

Direct from lava to clay
Obsidian Photo Gallery. Photo (c)2010 Andrew Alden, licensed to About.com ( fair use policy)

Water attacks obsidian readily because none of its material is locked up in crystals, making it prone to alteration into clays and related minerals.

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Weathered Obsidian

Delicate natural sculpture
Obsidian Photo Gallery. Photo (c) 2010 Andrew Alden, licensed to About.com ( fair use policy)

Like a sculptor grinding and brushing away the grit, wind and water have etched out subtle details inside this obsidian cobble from Glass Buttes, Oregon.

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Obsidian Tools

Good enough to work
Obsidian Picture Gallery. Photo (c) 2008 Andrew Alden, licensed to About.com ( fair use policy)

Obsidian is the best material for making stone tools. The stone doesn't need to be perfect to make useful implements.

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Obsidian of Glass Buttes, Oregon

Possibly work fragments
Gallery of Obsidian. Photo (c) 2010 Andrew Alden, licensed to About.com ( fair use policy)

Obsidian fragments from a few square meters show the full range of its typical textures and colors. The piece on the right appears to be a tool. Perhaps this spot was a workshop.

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Obsidian Chips

Glorious rubbish
Obsidian Picture Gallery. Photo (c) 2006 Andrew Alden, licensed to About.com ( fair use policy)

These chips, collectively called debitage, are from a prehistoric work site in eastern California. They display some of the variety in obsidian's color and transparency.

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Alden, Andrew. "A Look at the Multitude of Variations of Obsidian Rock." ThoughtCo, Oct. 24, 2017, thoughtco.com/pictures-of-obsidian-4123014. Alden, Andrew. (2017, October 24). A Look at the Multitude of Variations of Obsidian Rock. Retrieved from https://www.thoughtco.com/pictures-of-obsidian-4123014 Alden, Andrew. "A Look at the Multitude of Variations of Obsidian Rock." ThoughtCo. https://www.thoughtco.com/pictures-of-obsidian-4123014 (accessed May 27, 2018).