Mineral byproduct (anthropogenic / techno-fossil)
Iron Slag
Vitreous glassy waste (Fe-Si-Al-Ca compounds)
Hardness: 6-7 on Mohs scale; Color: Gray, black, or earthy brown with possible reddish oxidation; Luster: Dull to vitreous (glass-like) in broken areas; Structure: Amorphous/microcrystalline with a vesicular (pitted) texture; Specific Gravity: 2.8-3.5.
- Hardness
- 6-7 on Mohs scale
- Color
- Gray, black, or earthy brown with possible reddish oxidation
- Luster
- Dull to vitreous (glass-like) in broken areas
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Physical properties
Hardness: 6-7 on Mohs scale; Color: Gray, black, or earthy brown with possible reddish oxidation; Luster: Dull to vitreous (glass-like) in broken areas; Structure: Amorphous/microcrystalline with a vesicular (pitted) texture; Specific Gravity: 2.8-3.5.
Formation & geological history
Formed as a byproduct of the iron smelting process where impurities (silica, alumina, calcium) are separated from the molten iron. It is an industrial leftover commonly dating from the Industrial Revolution (18th-20th centuries) to modern times.
Uses & applications
Historically used as railroad ballast, road base, and aggregate in concrete. It has low value for jewelry but is occasionally used by collectors of 'industrial history' or as aquarium decor.
Geological facts
Slag can often be mistaken for meteorites or volcanic rocks (scoria/pumice) because of its vesicular texture. Over centuries, these 'techno-fissils' have become part of the geological record in former industrial heartlands.
Field identification & locations
Identify in the field by looking for gas bubbles (vesicles), a glass-like fracture pattern, and its typical proximity to old railways, canals, or blast furnace sites. Unlike meteorites, it usually has a 'cinder' look and does not have a fusion crust.
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