igneous
Slag (Industrial Glass/Glassy Rock)
Anthropogenic Vitreous Material
Hardness: 5-6 (Mohs scale); Color: Dark grey to black with brownish rust spots; Luster: Vitreous to dull; Crystal structure: Amorphous (non-crystalline); Cleavage: Conchoidal; Features: Vesicular (gas bubbles) and flow textures.
- Hardness
- 5-6 (Mohs scale)
- Color
- Dark grey to black with brownish rust spots
- Luster
- Vitreous to dull
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Physical properties
Hardness: 5-6 (Mohs scale); Color: Dark grey to black with brownish rust spots; Luster: Vitreous to dull; Crystal structure: Amorphous (non-crystalline); Cleavage: Conchoidal; Features: Vesicular (gas bubbles) and flow textures.
Formation & geological history
Formed as a byproduct of smelting ores (iron, steel, or copper) or from high-heat industrial processes. While technically man-made, it is chemically and structurally similar to volcanic rocks like obsidian or scoria, often found in historic industrial sites or roadbeds.
Uses & applications
Historically used as railroad ballast, road base material, or even in construction of buildings. It is now primarily a curiosity for rockhounds or recycled in cement production.
Geological facts
Slag is often mistaken for meteorites due to its dark, fused-looking surface and potential magnetism; however, the presence of spherical gas bubbles (vesicles) is a definitive indicator of slag or terrestrial lava rather than meteorites.
Field identification & locations
Identify in the field by looking for gas bubbles, glassy texture on fresh breaks, and proximity to historic railroads or industrial sites. It can be found globally in areas with previous mining or smithing activity.
