igneous (artificial/anthropogenic)

Slag (Industrial Byproduct)

Ferrous/Non-ferrous Silicate

Hardness: 5-7 (Mohs); Color: Black, brown, and metallic grey with yellowish weathering; Luster: Glassy to metallic; Crystal Structure: Amorphous (glassy); Features: Vesicular (full of air bubbles or gas pockets), conchoidal fracture.

Hardness: 5-7 (Mohs)
Color: Black, brown, and metallic grey with yellowish weathering
Luster: Glassy to metallic

Identified April 6, 2026More igneous (artificial/anthropogenic) →

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Physical properties

Formation & geological history

Formed as a byproduct of smelting ores to extract metals like iron, copper, or lead. It is a glass-like mass left over after the desired metal has been separated. While not a natural stone, it is a common geological find in historical industrial areas.

Uses & applications

Historical slag is often discarded. Modern slag is used in road base construction, as an aggregate in concrete, or in the manufacturing of mineral wool insulation and phosphate fertilizers.

Geological facts

Slag is often mistaken for meteorites due to its dark color, metallic sheen, and occasional magnetic properties. However, its high concentration of vesicles (bubbles) is a primary indicator that it is terrestrial industrial waste.

Field identification & locations

Identify by looking for spherical bubbles (vesicles), a glassy texture, and presence near railroads or old foundry sites. Collectors should check for magnetism; if it's very light despite looking metallic, it's likely slag.

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