Antropogenic Glass / Mineraloid
Ferruginous Slag (Iron Smelting Byproduct)
Fused Silicate Scoria (Amorphous mixture of FeO, SiO2, and CaO)
Hardness: 5-6 Mohs. Color: Charcoal grey to reddish-brown. Luster: Dull to sub-metallic with glassy inclusions. Structure: Vesicular (contains air bubbles/voids) and amorphous. Specific Gravity: Variable (2.8-4.0).
- Hardness
- 5-6 Mohs
- Color
- Charcoal grey to reddish-brown
- Luster
- Dull to sub-metallic with glassy inclusions
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Physical properties
Hardness: 5-6 Mohs. Color: Charcoal grey to reddish-brown. Luster: Dull to sub-metallic with glassy inclusions. Structure: Vesicular (contains air bubbles/voids) and amorphous. Specific Gravity: Variable (2.8-4.0).
Formation & geological history
Formed as a byproduct of iron or copper smelting. It is the waste material resulting from the melting of ores mixed with flux (limestone). While not a 'natural' rock, these specimens can range from the Roman era to the Industrial Revolution.
Uses & applications
Historically used as road ballast, railway bedding, and in some types of concrete cement. Modern collectors value it for its historical industrial context or unique 'space-rock' aesthetic.
Geological facts
Slag is often mistaken for meteorites due to its dark color, high density, and occasional magnetic properties. However, its internal air bubbles (vesicles) are a definitive indicator of terrestrial cooling at the surface.
Field identification & locations
Identify in the field by looking for spherical bubbles (vesicles), a high weight relative to size (high SG), and sharp, glass-like edges when broken. It is extremely common in areas with a history of iron working or near old railway lines.
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