The Many Variations of Obsidian Rock

Close up of a Native American obsidian arrowhead in the dirt.

Tyler Hulett/Getty Images

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 lava very high in silica (more than about 70 percent), such as rhyolite. The many strong chemical bonds between silicon and oxygen make such 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 low water content may inhibit crystallization. View pictures of obsidian in this gallery.

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Obsidian Flow

Large obsidian flow on a bright, sunny day, with rolling hills in the background.

daveynin/Flickr/CC BY 2.0

Big obsidian flows display the rugged surface of the highly viscous lava that forms obsidian.

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Obsidian Blocks

Large obsidian rock formations.

GarysFRP/Getty Images

Obsidian flows develop a blocky surface as their outer shell quickly solidifies.

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Obsidian Flow Texture

Obsidian flow formation against a cloudless blue sky.

TheCADguy/Pixabay

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

Obsidian flow containing spherulites.

James St. John/Flickr/CC BY 2.0

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

Chunk of obsidian in a rocky landscape on a sunny day.

Rosmarie Wirz/Getty Images

Usually black, obsidian can also be red or gray, streaked and mottled, and even clear.

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Obsidian Cobble

Chunk of obsidian rock pictured with a coin for scale on a neutral background.

ThoughtCo/Andrew Alden

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

Obsidian chunk displaying two different textures, rough and smooth, with a coin for scale on a neutral background.

ThoughtCo/Andrew Alden

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

In some obsidian pieces, 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 age of 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. Therefore, they bear cultural as well as geologic information.

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Weathering of Obsidian

Chunk of obsidian lava next to a coin for scale on a dark gray background.

ThoughtCo/Andrew Alden

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

Chunk of weathered obsidian on a carpet.

Teravolt (talk · contribs)/Wikimedia Commons/CC BY 3.0

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

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Obsidian Tools

Obsidian spear points on a yellow background.

Simon Evans - simon@cognable.com/Wikimedia Commons/CC BY 3.0

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 Fragments

Shards of obsidian on a white background.

James St. John/Flickr/CC BY 2.0

Obsidian fragments show the full range of its typical textures and colors.

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Obsidian Chips

Obsidian chips and fragments in different plastic containers.

Zde/Wikimedia Commons/CC BY 4.0

These chips are collectively called debitage. They display some of the variety in obsidian's color and transparency.