industrial byproduct (anthropogenic)
Iron Slag
Ferrous Slag (Silicate/Oxide mixture)
Hardness: 5-7 (Mohs scale), Color: Dark grey to black with occasional iridescent blue/purple sheen, Luster: Vitreous to metallic, Structure: Amorphous or glassy, Cleavage: None/Conchoidal, Specific Gravity: 2.5-4.5 (varies by iron content)
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Physical properties
Hardness: 5-7 (Mohs scale), Color: Dark grey to black with occasional iridescent blue/purple sheen, Luster: Vitreous to metallic, Structure: Amorphous or glassy, Cleavage: None/Conchoidal, Specific Gravity: 2.5-4.5 (varies by iron content)
Formation & geological history
Formed as a byproduct of the smelting process when iron ore is separated from its impurities. It is common at historical industrial sites, railroads, and former blast furnaces.
Uses & applications
Historically a waste product, but now used in road construction, as ballast, as an ingredient in Portland cement, and sometimes as a base material for mineral wool insulation.
Geological facts
Slag is often mistaken for meteorites due to its dark, fused appearance and occasional magnetism, but its vesicular (bubbly) texture is a key indicator of its terrestrial, industrial origin.
Field identification & locations
Identify in the field by looking for gas bubbles (vesicles), a glassy/scoriaceous texture, and heavy weight relative to size. It is commonly found near old railways and steel mills.
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