Mineraloid / Anthropogenic
Slag
Ferrous slag (by-product of iron smelting)
Hardness: 5-7 Mohs; Color: Black, dark grey, or brownish-black; Luster: Glassy to metallic; Structure: Amorphous/Vesicular; Cleavage: None (conchoidal fracture); Specific Gravity: 2.8 - 4.5 depending on metal content.
- Hardness
- 5-7 Mohs
- Color
- Black, dark grey, or brownish-black
- Luster
- Glassy to metallic
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Physical properties
Hardness: 5-7 Mohs; Color: Black, dark grey, or brownish-black; Luster: Glassy to metallic; Structure: Amorphous/Vesicular; Cleavage: None (conchoidal fracture); Specific Gravity: 2.8 - 4.5 depending on metal content.
Formation & geological history
Formed as a stony waste matter separated from metals during the smelting or refining of ore. Most specimens found in modern environments date from the Industrial Revolution (19th century) to the present day. It is often found near old blast furnaces or railroad beds.
Uses & applications
Used as railroad ballast, road base material, and occasionally processed into mineral wool insulation. Smaller, colorful pieces are sometimes sold to novice collectors as 'meteorites' or industrial curiosities.
Geological facts
Slag is frequently mistaken for meteorites due to its dark color and occasional magnetism. However, the presence of spherical gas bubbles (vesicles) is a definitive indicator that it is terrestrial slag rather than a meteorite, as meteorites do not contain bubbles.
Field identification & locations
Identify by looking for glass-like texture mixed with vesicles (air holes). It is commonly found in areas of historical industrial activity. Unlike lava (scoria), slag often has a higher density and may contain small inclusions of pure metal or glass.
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