industrial byproduct (anthropogenic)
Iron Slag
Ferrous Slag (complex silicate and oxide byproduct)
Hardness: 5-7 (Mohs); Color: Dark grey to black with reddish rust (iron oxide) staining; Luster: Vitreous to dull/earthy; Crystal Structure: Amorphous (glassy); Cleavage: None/Conchoidal fracture; Presence of spherical vesicles (gas bubbles); Specific Gravity: Variable (2.5 - 4.0 depending on iron content).
- Hardness
- 5-7 (Mohs)
- Color
- Dark grey to black with reddish rust (iron oxide) staining
- Luster
- Vitreous to dull/earthy
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Physical properties
Hardness: 5-7 (Mohs); Color: Dark grey to black with reddish rust (iron oxide) staining; Luster: Vitreous to dull/earthy; Crystal Structure: Amorphous (glassy); Cleavage: None/Conchoidal fracture; Presence of spherical vesicles (gas bubbles); Specific Gravity: Variable (2.5 - 4.0 depending on iron content).
Formation & geological history
Formed as a byproduct of the smelting process when iron ore is separated from impurities (siliceous rock and limestone) in a blast furnace. Though modern slag exists, many specimens found in the forest or near railroads are historical remnants from the 19th or early 20th century.
Uses & applications
Used primarily in construction for road base, railway ballast, as an aggregate in concrete, and for abrasive grit in sandblasting. Historical slag is mostly considered industrial waste but used today for eco-friendly building materials.
Geological facts
Slag is often mistaken for meteorites due to its dark color, high density, and occasional magnetic properties; however, the presence of numerous circular vesicles (gas bubbles) as seen in the image is a definitive sign of terrestrial slag.
Field identification & locations
Identify in the field by searching for 'Swiss cheese' texture (vesicles), high density, and proximity to old railway lines, mines, or industrial sites. Collectors often find it as an interesting curiosity or 'meteorwrong.'
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