mineral

Slag (Industrial By-product)

Anthropogenic Vitreous Silica/Iron Silicate

Hardness: 5.5-7 (variable); Color: Black to dark brown; Luster: Vitreous (glassy) to dull; Crystal Structure: Amorphous (non-crystalline); Cleavage: Conchoidal fracture; Specific Gravity: 2.5-4.0

Hardness: 5
Color: Black to dark brown
Luster: Vitreous (glassy) to dull

Identified May 7, 2026More mineral →

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Physical properties

Hardness: 5.5-7 (variable); Color: Black to dark brown; Luster: Vitreous (glassy) to dull; Crystal Structure: Amorphous (non-crystalline); Cleavage: Conchoidal fracture; Specific Gravity: 2.5-4.0

Formation & geological history

Produced as a glass-like by-product of smelting ores to extract metals like iron, copper, or lead. It forms when the stony waste matter (flux) is separated from the raw metal during the heating process. Found globally in historical and modern industrial sites.

Uses & applications

Historically a waste product, but modern uses include road construction material, railway ballast, concrete aggregate, and sometimes as an abrasive sandblasting agent.

Geological facts

Slag can often be mistaken for obsidian or meteorites due to its glassy appearance and occasional presence of gas bubbles (vesicles). It is technically a 'technofossil' representing human industrial history.

Field identification & locations

Identify by looking for spherical gas bubbles (vesicles), flow lines, and its proximity to historical railroads or foundry sites. It may be slightly magnetic depending on iron content. It is most commonly found in old industrial regions or along train tracks.

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