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
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Physical properties
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.
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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