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Obsidian Collection — Identification and Extraction of Volcanic Glass in Mexico
English
SeñorMartillo

Created by

SeñorMartillo

23. April 2026MX
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Obsidian Collection — Identification and Extraction of Volcanic Glass in Mexico

Detailed guide to identifying, locating, and collecting obsidian from volcanic deposits in Mexico. Obsidian is a natural volcanic glass formed when felsic lava cools rapidly, preventing crystal structure formation. Mexico possesses some of the world's finest obsidian deposits.

Intermediate
Full day field trip

Instructions

1

Understanding the Geology of Obsidian

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.
2

Identifying Volcanic Terrain and Known Deposits

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).

Materials for this step:

Mapa Geológico RegionalMapa Geológico Regional1 piece
3

Safety Preparation for Fieldwork

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.

Materials for this step:

Botiquín de Primeros AuxiliosBotiquín de Primeros Auxilios1 piece
Agua PotableAgua Potable3 liters
Protector SolarProtector Solar1 piece

Tools needed:

Guantes de Cuero GruesoGuantes de Cuero Grueso
Botas de Suela GruesaBotas de Suela Gruesa
4

Surface Prospecting Techniques

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.

Tools needed:

Varilla de Sondeo (1 metro)Varilla de Sondeo (1 metro)
5

Identifying Genuine Obsidian — Field Tests

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.
6

Recognizing Obsidian Varieties by Color

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.
7

Careful Extraction of Specimens

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.

Tools needed:

Cincel de GeólogoCincel de Geólogo
Martillo de GeólogoMartillo de Geólogo
Guantes de Cuero GruesoGuantes de Cuero Grueso
8

Field Cleaning of Specimens

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.

Materials for this step:

Papel PeriódicoPapel Periódico10 sheets
Paño de Algodón SuavePaño de Algodón Suave2 pieces

Tools needed:

Cepillo de Cerdas SuavesCepillo de Cerdas Suaves
9

Specimen Quality Assessment

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.
10

Documenting the Find Location

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).

Materials for this step:

Cuaderno de CampoCuaderno de Campo1 piece

Tools needed:

Teléfono con GPSTeléfono con GPS
Regla de Referencia (15 cm)Regla de Referencia (15 cm)
11

Labeling Specimens

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.

Materials for this step:

Etiquetas de Papel ResistenteEtiquetas de Papel Resistente20 pieces
Lápiz de GrafitoLápiz de Grafito1 piece
Esmalte Blanco (corrector líquido)Esmalte Blanco (corrector líquido)1 piece
12

Safe Transport and Storage

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.

Materials for this step:

Caja de Cartón Resistente con DivisionesCaja de Cartón Resistente con Divisiones1 piece
Espuma de Polietileno para AcolchamientoEspuma de Polietileno para Acolchamiento1 kilogramo

Materials

12

Tools Required

8

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