Anthropogenic (Man-made industrial byproduct)
Slag
Anthropogenic Vitrification Product / Silicate Slag
Hardness: 5-6 (Mohs scale); Color: Black, charcoal gray, or dark brown; Luster: Vitreous (glassy) to dull; Crystal structure: Amorphous (glass-like); Common features: Vesicular (contains air bubbles/gas pockets), flow textures, or metallic inclusions; Specific gravity: 2.5-4.5 (varies by metal content).
- Hardness
- 5-6 (Mohs scale)
- Color
- Black, charcoal gray, or dark brown
- Luster
- Vitreous (glassy) to dull
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Physical properties
Hardness: 5-6 (Mohs scale); Color: Black, charcoal gray, or dark brown; Luster: Vitreous (glassy) to dull; Crystal structure: Amorphous (glass-like); Common features: Vesicular (contains air bubbles/gas pockets), flow textures, or metallic inclusions; Specific gravity: 2.5-4.5 (varies by metal content).
Formation & geological history
Formed during metal smelting (modern or historical). It is a byproduct created when ore is heated and the rocky impurities (silica, alumina) melt and separate from the liquid metal. Once cooled, it forms a hard, glass-like mass.
Uses & applications
Historically discarded as waste. Modern uses include road construction base material, railroad ballast, or as an ingredient in cement. Some unusual specimens are used in jewelry (like 'Leland Blue' or 'Sieber Agate') as decorative glass-like stones.
Geological facts
Slag is the most common 'meteor-wrong'—it is frequently mistaken for meteorites due to its dark color, weight, and irregular shape. However, the presence of spherical gas bubbles (vesicles) is a definitive indicator of slag, as true meteorites do not contain air bubbles.
Field identification & locations
Identify in the field by checking for vesicles (bubbles), which are rare in most natural rocks but common in slag. It is often found near old railroad tracks, former iron foundries, or historic mining sites. Often more magnetic than surrounding rocks due to residual iron content.
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