mineraloid / anthropogenic
Slag (Industrial Glass Slag)
Amorphous Silica / Ferro-silicate Slag
Hardness: 5.5-6.5 (Mohs scale); Color: Olive green, dark gray, or black; Luster: Vitreous to dull; Crystal structure: Amorphous (non-crystalline); Cleavage: Conchoidal fracture; Specific gravity: 2.5-4.0 (varies based on metal content).
- Hardness
- 5
- Color
- Olive green, dark gray, or black
- Luster
- Vitreous to dull
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Physical properties
Hardness: 5.5-6.5 (Mohs scale); Color: Olive green, dark gray, or black; Luster: Vitreous to dull; Crystal structure: Amorphous (non-crystalline); Cleavage: Conchoidal fracture; Specific gravity: 2.5-4.0 (varies based on metal content).
Formation & geological history
Formed as a byproduct of smelting ores (iron, copper, or lead) or glass manufacturing. The molten waste cools rapidly into a glassy or stony mass. Found in areas with historical industrial or milling activity.
Uses & applications
Used historically in road construction, railroad ballast, and concrete aggregate. Green varieties are sometimes used in landscaping, aquarium decor, or as 'cullet' for jewelry making.
Geological facts
Slag is often mistaken for meteorites or rare minerals like obsidian or moldavite. Its distinctive 'vesicular' texture (air bubbles or 'craters') is a key indicator of its industrial origin.
Field identification & locations
Identify in the field by looking for air bubbles, flow patterns, or a glassy fracture on broken edges. It is often found near old railways or ironworks. It is non-magnetic unless it has a high iron content.
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