Mineral-like byproduct
Slag
Anthropogenic Vitrophyre (Iron or Smelting Slag)
Hardness: 5-6 (Mohs); Color: Dark brown, black, or grey; Luster: Glassy to metallic; Crystal Structure: Amorphous/glassy; Cleavage: None (conchoidal fracture); Specific Gravity: 2.5-4.5 depending on metal content.
- Hardness
- 5-6 (Mohs)
- Color
- Dark brown, black, or grey
- Luster
- Glassy to metallic
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Physical properties
Hardness: 5-6 (Mohs); Color: Dark brown, black, or grey; Luster: Glassy to metallic; Crystal Structure: Amorphous/glassy; Cleavage: None (conchoidal fracture); Specific Gravity: 2.5-4.5 depending on metal content.
Formation & geological history
Formed as a byproduct of historical or industrial smelting. When ore is heated, impurities separate into a molten silicate liquid that cools rapidly into a glass-like or stony material. Some pieces may be hundreds of years old from colonial forges.
Uses & applications
Historically a waste product; modernly used as road ballast, railway track bedding, and in certain types of concrete or insulation (rock wool). Collectors value it for historical context or aesthetic 'desert glass' appearance.
Geological facts
Slag is often mistaken for meteorites due to its dark, melted appearance and occasional magnetic properties, but the presence of vesicles (air bubbles) almost always identifies it as terrestrial slag.
Field identification & locations
Identify by looking for spherical air bubbles (vesicles), glassy texture, and high density. Commonly found near old railroad tracks, historical industrial sites, and iron bogs.
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