igneous
Slag (Industrial Byproduct)
Anthropogenic Vitreous Silicate material
Hardness: 5-6.5 Mohs. Color: Typically black, dark grey, or brownish-black with a glassy to metallic luster. Crystal structure: Amorphous/Vitreous (non-crystalline). Cleavage: None, exhibits conchoidal fracture. Often contains spherical gas bubbles (vesicles).
- Hardness
- 5-6
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Physical properties
Hardness: 5-6.5 Mohs. Color: Typically black, dark grey, or brownish-black with a glassy to metallic luster. Crystal structure: Amorphous/Vitreous (non-crystalline). Cleavage: None, exhibits conchoidal fracture. Often contains spherical gas bubbles (vesicles).
Formation & geological history
Slag is an anthropogenic geological material formed as a byproduct of smelting ores (iron, steel, copper) to separate the metal from unwanted stone. It forms when molten waste cools rapidly, creating a glass-like or stony solid. It is technically an artificial igneous rock.
Uses & applications
Commonly used as road base, railroad ballast, aggregate in concrete, and historically for landfill or glass-making. Some colorful varieties (like Leland Blue) are used in jewelry.
Geological facts
Because it looks so much like a natural volcanic rock or meteorite, slag is one of the most common 'false-positive' specimens brought to geologists. It can often be identified by the presence of perfectly round air bubbles.
Field identification & locations
Identify in the field by checking for vesicles (bubbles), a glassy texture, or metallic inclusions. It is frequently found near historical industrial sites, railroads, and old foundries. It is often non-magnetic unless it contains high concentrations of iron oxide.
