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Obsidian, photo 1
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Rock type

Obsidian

Also called Volcanic glass, Natural glass

Obsidianobsidian (volcanic glass)All regions

2 photos

Description

Obsidian is natural volcanic glass, formed when lava, usually rhyolite, cools too fast for crystals to grow. It is typically black, but red, brown, green and clear pieces occur, and banding or a reflective sheen sometimes shows. It breaks with a very sharp conchoidal edge. Researchers use X-ray fluorescence to trace artifacts to their source, and Oregon alone has more than 100 geochemical sources. Because the source matters, obsidian pages here are by source. Obsidian Cliff in Yellowstone is the most widely traded: its glass reached Ohio Hopewell sites about 1,800 years ago. Glass Buttes in Oregon was used for at least 14,000 years.

How to identify it

Colour
Typically black; sometimes clear, greenish or reddish (Bakken); red, brown or green hues come from the oxidation state of iron minerals; banding from oxidised flow surfaces folded into the lava (Ozbun).
Texture
Non-crystalline glass (holohyaline), without grains.
Luster
Glassy; stretched gas bubbles can give a reflective sheen or cat's-eye effect.
Inclusions
A tiny amount of finely dispersed magnetite gives the black colour.

Geology

Obsidian is the glassy texture of volcanic rock cooled too quickly to crystallise, and it can be basaltic, andesitic, dacitic or rhyolitic depending on silica. The most common kind is rhyolitic obsidian (68 to 77 percent silica) from viscous lava flows and domes. Basaltic glass, tachylyte, forms thin rinds on basalt flows. Obsidian also occurs in pyroclastic ash flows. Obsidian Cliff in Yellowstone is a 98-foot-thick exposure of glass; Glass Buttes is a large rhyolitic volcanic center about five million years old in south-central Oregon.

Where it comes from

All regions

States

Quarries and outcrops

  • Obsidian Cliff, Yellowstone National Park, Wyoming (more than 11,000 years of quarrying; National Historic Landmark 1996)
  • Glass Buttes, Lake County, Oregon (nine geochemical varieties; used for at least 14,000 years)
  • Obsidian sources of the Great Basin, such as those in Nevada and California listed by the Nevada Department of Transportation project

Used by

  • Obsidian Cliff was first quarried more than 11,000 years ago, and its glass reached Ohio Hopewell sites about 1,850 to 1,750 years ago (NPS)
  • Glass Buttes: at least fourteen millennia of use (Stueber and Skinner)

Look-alikes

Stones that are easy to confuse with Obsidian.

  • Basaltic glass (tachylyte) and glass in ash-flow tuffs, which are glassy but not rhyolitic obsidian (Ozbun)
  • Fine-grained rhyolite (aphanitic, not glassy)

Sources

  1. Ozbun, Terry L. 2015. Toolstone Geography and the Larger Lithic Landscape. Chapter 1 in Toolstone Geography of the Pacific Northwest. Archaeology Press, Simon Fraser University.archpress.lib.sfu.ca (opens in a new tab)
  2. Stueber, Daniel O., and Craig E. Skinner. 2015. Glass Buttes, Oregon: 14,000 Years of Continuous Use. Chapter 12 in Toolstone Geography of the Pacific Northwest. Archaeology Press, Simon Fraser University.archpress.lib.sfu.ca (opens in a new tab)
  3. National Park Service. Obsidian Cliff. Yellowstone National Park.nps.gov (opens in a new tab)
  4. Moore, Joe. 2009. Great Basin Tool-Stone Sources: The NDOT Obsidian and Tool-Stone Sourcing Project, 2002 Progress Report. Nevada Department of Transportation, Carson City.deschutesmeridian.com (opens in a new tab)
  5. Bakken, Kent E. (written about 1995; PDF issued later). Chipped Stone Raw Materials. Chapter 6 in Stone Tools of Minnesota. Minnesota Office of the State Archaeologist, St. Paul.mn.gov (opens in a new tab)

Named stones of this kind

Points made from this stone

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