igneous
Industrial Slag
Anthropogenic Vitreous Slag
Hardness: 5-7 (Mohs); Color: Dark gray, black, or blue-ish with metallic sheen; Luster: Glassy to metallic; Structure: Vesicular (contains air bubbles/pores) and amorphous; Specific Gravity: Varies widely based on metal content.
- Hardness
- 5-7 (Mohs)
- Color
- Dark gray, black, or blue-ish with metallic sheen
- Luster
- Glassy to metallic
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Physical properties
Hardness: 5-7 (Mohs); Color: Dark gray, black, or blue-ish with metallic sheen; Luster: Glassy to metallic; Structure: Vesicular (contains air bubbles/pores) and amorphous; Specific Gravity: Varies widely based on metal content.
Formation & geological history
Formed as a byproduct of smelting ores (iron, steel, or copper) or from coal combustion in industrial furnaces. It is technically an 'anthropogenic' igneous rock, cooling from a molten state created by human activity rather than volcanic activity.
Uses & applications
Used as a base material in road construction, as railroad ballast, in asphalt, or as grit for sandblasting. Some unique 'glassy' slag is collected as 'Leland Blue' or 'Sieber Agate' for lapidary use.
Geological facts
Slag is often mistaken for meteorites due to its dark color and metallic appearance, but the presence of vesicles (bubbles) almost always identifies it as slag or terrestrial basalt. It is one of the most common 'human-made' rocks found worldwide.
Field identification & locations
Identify in the field by looking for gas bubbles (vesicles), a glassy texture, or unusual colors like bright blue or green. It is often found near old railroad tracks, former industrial sites, or in fill dirt.
