Obsidian, a form of natural volcanic glass, is a highly sought-after material for collectors due to its sharp conchoidal fracture and unique glassy appearance. This material is not a true mineral because it lacks a crystalline structure, but its formation requires a specific and rapid cooling process. Arizona is one of the premier locations in the United States for finding this material, with a geological history that has produced numerous accessible deposits.
The Volcanic Origins of Arizona Obsidian
Arizona’s abundance of obsidian is directly linked to the extensive volcanic activity that shaped the northern and central portions of the state. The San Francisco Volcanic Field, located near Flagstaff, is the primary geological source for much of the region’s volcanic glass. This field contains hundreds of extinct volcanoes and domes, where silica-rich lava, known as rhyolite, cooled quickly. Obsidian forms when this felsic lava solidifies so rapidly that mineral crystals do not have time to grow.
The volcanic structures in the San Francisco field range in age, with sources like Government Mountain dating back nearly 2.7 million years. Obsidian often occurs as nodules within the alluvium or eroding from the base of these rhyolite domes. This process disperses the glass, making it available for surface collection across the surrounding plateau.
Key Public Access Collection Areas
One of the most productive sites is the area surrounding Government Mountain, located northwest of Flagstaff in the Kaibab National Forest. Here, collectors can find black obsidian nodules, some reaching up to 15 centimeters in diameter, particularly in the vicinity of the Obsidian Tank area.
For those seeking a specific variety, the area near Superior in Pinal County is famous for its Apache Tears. These small, rounded pieces of obsidian are often discovered loose on the ground or embedded in a whitish volcanic rock matrix called perlite. Another location for these small nodules is the Burro Creek area in western Arizona, near the U.S. 93 bridge. The obsidian found there is often a weathered black glass with a brown or gray interior.
Identifying Arizona’s Unique Obsidian Varieties
Arizona’s volcanic history has produced several visually distinct types of obsidian, each identifiable by specific characteristics. Apache Tears are perhaps the most famous, appearing as small, dark, rounded nodules. They are often opaque black but become translucent and smoky brown when held up to a strong light source. They are created when lava is thrown into the air during an eruption, rapidly cooling before hitting the ground.
Mahogany Obsidian displays reddish-brown streaks or patches within the typical black glass. This coloration is caused by the inclusion of small amounts of iron oxide, which forms during the cooling process. Snowflake Obsidian is characterized by white, radially clustered crystals of cristobalite embedded in the black glass matrix. These white inclusions create a distinctive pattern resembling snowflakes, formed by the partial devitrification of the glass over time.
Regulations for Rock Hounding and Collection
Collecting obsidian in Arizona requires adherence to specific regulations depending on the type of land. On Bureau of Land Management (BLM) administered public lands, individuals may collect specimens for personal use without a permit. The limit is defined as up to 25 pounds per person per day, with a total annual limit of 250 pounds.
Any collection must be done using hand tools only; the use of mechanical equipment or explosives is prohibited. On National Forest System lands, such as the Kaibab National Forest, personal-use collection is often allowed, though a free permit may be required in some areas. It is prohibited to collect any material on State Trust Land, in National Parks, or in National Monuments. Furthermore, removing archaeological artifacts, such as arrowheads or pottery shards, is illegal on all federal lands, regardless of the property’s designation.