igneous (anthropogenic)

Slag (Industrial Glass/Byproduct)

Anthropogenic Vitreous Silicate

Hardness: 5.5-6.5 (Mohs scale); Color: Dark grey, brownish, or metallic; Luster: Glassy to dull; Crystal structure: Amorphous/Vitreous; Cleavage: None/Conchoidal fracture; Contains vesicles (gas bubbles).

Hardness: 5
Color: Dark grey, brownish, or metallic
Luster: Glassy to dull

Identified April 2, 2026More igneous (anthropogenic) →

Identify your own rocks.

Get a report just like this from any photo, free.

App Store Google Play

Physical properties

Formation & geological history

Formed as a byproduct of smelting ores (like iron, copper, or lead) where impurities are separated from the metal. Molten slag cools rapidly on the surface or in piles, creating a glass-like or stony texture. These can date from the Industrial Revolution to modern times.

Uses & applications

Historically used for road ballast, railroad beds, and occasionally as an aggregate in concrete. It is also a popular 'curiosity' item for amateur rockhounds who misidentify it as meteorites.

Geological facts

Slag is often the most common 'meteor-wrong' found by collectors because of its metallic appearance, heavy weight, and presence of gas bubbles (vesicles), which real meteorites almost never have.

Field identification & locations

Identify by looking for spherical gas bubbles (vesicles), a glassy texture upon breaking, and flow lines on the surface. It is commonly found near old railways, industrial sites, or historic furnace locations.

Other igneous (anthropogenic) specimens

See all

Green Slag Glass

Amorphous Silicon Dioxide (with metallic impurities)

Igneous (Anthropogenic)

Ferrocoke or Slag

Anthropogenic pyrometallurgical residue

igneous

Lava Rock with Fabric Impression

Scoria or Basalt with anthropogenic impressions

igneous

Cupronickel (on a 1981 Mexico 20 Pesos coin)

Copper-Nickel Alloy (Cu75Ni25)

mineral

Basalt

Basalt (Mafic volcanic rock)

igneous

Basalt

Basalt (Mafic Extrusive Igneous Rock)

igneous