Hollister Field Office

Detail of obsidian rock sample, photo by E. Zaborsky BLMObsidian Artifacts Studies

Detail of raw obsidian sample, photo by E. Zaborsky BLM

Close-up detail of raw obsidian rock material

In 2011 the BLM Hollister Field Office began a comprehensive review of obsidian artifacts collected since the late 1970's.  This review included determining the geologic source of the artifacts through x-ray fluorescence (XRF) analysis and measurement of hydration rims that are created over time as the artifact absorbs water from the environment.

What is obsidian? - From the United States Geological Survey Volcanoes Hazards Program glossary

What is XRF? - A good discussion of x-ray fluorescence from Carleton College, Science Education Resource Center at Montana State University and National Science Foundation

What is obsidian hydration (OH)? - Quick definition from the National Park Service

Fire Regimes and Fire History: Implications for Obsidian Hydration Dating (USFS 2002)

Projectile point, photo by BLM

Bottom (distal) end of Stockton serrated formed obsidian projectile point discovered during prescribed fire operations in San Benito County, California

Projectile point, photo by BLMPetryglyph, photo by BLMHistoric structures, photo by BLM

BLM Cultural Resources

Projectile point, photo by BLM

Obsidian Projectile Point

International Association for Obsidian Studies

Projectile point, photo by E. Zaborsky BLM

Contracting stem obsidian projectile point from San Benito County, California

Map of Obsidian Sources in California


The majority of artifacts sampled came from prehistoric archeological sites located in Fresno and San Benito Counties.  Preliminary results indicate that the predominant (or "preferred") source location that Native American California Indians utilized for obsidian stone tools was in present-day Inyo County at the Coso Mountains.  The second most 'popular' source for obsidian to make artifacts also came from a trans-Sierran source called Casa Diablo located in present-day Mono County.  The remaining 20-30% of the obsidian source material comes from the northern California coastal mountains in present day Lake and Napa Counties.

Obsidian studies in west-central California are important in understanding prehistoric archeological sites because there are no local sources of obsidian for people to quarry; this meant that people had to trade or travel over long distances (over 100 miles) to obtain this material.  One reason obsidian was favored as a lithic (stone) material for making sharp tools like projectile points (arrowheads) is because of the glass-like qualities of obsidian rock.

Bureau of Land Management
Hollister Field Office
20 Hamilton Court
Hollister, CA 95023
Phone: (831) 630-5000
Fax: (831) 630-5055
Office Hours: 7:30 a.m. - 4:00 p.m., M-F
Contact us by Email