Obsidian Collection — Identification and Extraction of Volcanic Glass in Mexico

Obsidian is a natural volcanic glass that forms when felsic (silica-rich) lava cools so rapidly that atoms cannot organize into a crystalline structure. The result is an amorphous solid with vitreous luster and conchoidal fracture. Typical composition: 70-75% SiO2, plus Al2O3, Na2O, K2O, and Fe3O4. Hardness is 5-5.5 on the Mohs scale. It forms at the margins of rhyolitic and dacitic lava flows. Mexico is exceptionally rich in obsidian due to intense volcanic activity along the Trans-Mexican Volcanic Belt.

Major Mexican obsidian deposits include: Sierra de las Navajas in Pachuca, Hidalgo — the most famous Mesoamerican deposit and primary source for Teotihuacan and the Aztecs; Otumba region, State of Mexico; deposits near Guadalajara, Jalisco; volcanic zones around Pico de Orizaba, Veracruz; and lava flows in Puebla and Michoacan. Obsidian is found on or near the surface in rhyolitic lava flow terrain. Study geological maps from the Mexican Geological Survey (SGM).

Obsidian is extremely sharp — fractured edges can be sharper than surgical steel (down to 30 angstroms). Wear thick leather gloves, thick-soled boots, long pants, and long-sleeved shirt. Pack a first-aid kit. Carry minimum 3 liters of water, sunscreen, hat, and food. Inform someone of your destination. Check weather — avoid volcanic terrain during heavy rains due to landslide risk.

Obsidian is frequently found as nodules and fragments on the surface near lava flows. Walk slowly, looking for vitreous (glass-like) fragments. Nodules are often partially buried — erosion constantly exposes new pieces. Check volcanic hillslopes, dry streambeds, and road cuts exposing subsurface layers. Use a light probing rod to check for nodules at 5-15 cm depth.

Three key tests: 1) Conchoidal fracture — obsidian breaks with curved, smooth surfaces producing sharp edges. 2) Vitreous luster — glass-like shine, not matte or earthy. 3) Translucency — thin edges transmit light, showing yellow to reddish-brown glow against the sun. Obsidian has no visible crystalline structure — visible crystals or grains indicate basalt or another crystalline rock.

Color varieties: Black — most common, from magnetite nanoparticles. Green — from low ferrous iron (Fe2+), abundant at Sierra de las Navajas; Pachuca green obsidian was most coveted by the Aztecs. Mahogany — reddish bands from hematite inclusions. Rainbow — iridescence from nanometric magnetite crystal layers. Snowflake — white cristobalite inclusions from partial devitrification. Golden sheen — golden reflection from aligned gas bubble inclusions.

Extract carefully to preserve integrity. Pick up loose surface fragments with leather gloves. For partially buried nodules, use a geologist's chisel and hammer to loosen surrounding soil without striking the obsidian directly. Work the chisel into soil, not rock. For large pieces (over 15 cm), dig a channel around the specimen. Never separate nodules from bedrock with hard blows — obsidian fractures conchoidally and can launch sharp fragments.

Clean specimens in the field using a soft-bristle brush (never metallic). For persistent dirt, use clean water with a soft cloth. Do not rub with abrasive materials — obsidian (5-5.5 Mohs) scratches easily. Examine under good light for clarity, color, inclusions, and vitreous luster. Specimens with intact natural fracture surfaces are more valuable. Wrap each individually in newspaper or soft cloth for transport.

Assess by: Size — pieces over 10 cm are more versatile. Homogeneity — uniform obsidian without large inclusions or internal fractures is higher quality; check against strong light. Color — green Pachuca and rainbow are most prized. Luster — intense vitreous luster indicates good preservation. Shape — rounded nodules with intact cortex are valuable. Obsidian absorbs water at a measurable rate (obsidian hydration), used archaeologically for dating.

Record exact GPS coordinates. Photograph specimens in situ with a scale reference. Photograph the geological surroundings. Note in field notebook: date, coordinates, altitude, terrain type, color and estimated size, and geological observations. If you find worked artifacts (arrowheads, scrapers, blades), do not remove them — they may be protected by INAH (National Institute of Anthropology and History).

Label each specimen with a unique number corresponding to your field notebook. Write on durable paper tags with pencil (ink erases with moisture). Note: specimen number, locality, date, color variety, and approximate weight/size. For formal collections, apply white enamel to a discreet area and write the number with indelible ink once dry.

Transport specimens individually wrapped in newspaper or cloth in a sturdy padded box. Do not stack without protection. Secure the box on the vehicle floor. For long-term storage, keep in a dry place away from direct sunlight. Display on padded bases. Obsidian is geologically stable and needs no special conservation. Collecting is legal in many Mexican localities, but archaeological sites are protected — consult INAH regulations.