Antropogenic Glass / Mineraloid

Ferruginous Slag (Iron Smelting Byproduct)

Fused Silicate Scoria (Amorphous mixture of FeO, SiO2, and CaO)

AI-generated identification · may be incorrect

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

Identified March 17, 2026More antropogenic glass / mineraloid →

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Physical properties

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.