igneous (artificial/anthropogenic)

Slag

Ferrous Slag (Amorphous calcium-magnesium-aluminum silicates)

Hardness: 5-7 (Mohs). Color: Dark brown to black with metallic sheen. Luster: Vitreous to sub-metallic. Structure: Vesicular (contains air bubbles/holes), non-crystalline/glassy. Cleavage: None (conchoidal fracture). Specific gravity: 2.5-3.0.

Hardness: 5-7 (Mohs)
Color: Dark brown to black with metallic sheen
Luster: Vitreous to sub-metallic

Identified January 5, 2026More igneous (artificial/anthropogenic) →

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Physical properties

Formation & geological history

Formed as a byproduct of smelting ores (iron or copper) during the 19th and 20th centuries. It is the stony waste matter separated from metals. Geological age: Anthropocene (modern industrial era).

Uses & applications

Used in modern construction for road base aggregate, railway ballast, concrete manufacturing, and as a raw material for mineral wool insulation. Historically discarded as waste.

Geological facts

Slag is often mistaken for meteorites due to its dark, iron-rich appearance and pits. However, the presence of vesicles (bubbles) is a primary indicator it is industrial man-made glass/slag rather than a space rock.

Field identification & locations

Identify by looking for glass-like sharp edges, spherical air bubbles, and a weight that is often lighter than iron ores but heavier than common rocks. Found near historical industrial sites, railroads, and riverbanks where waste was dumped.

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