mineraloid
Slag (Industrial Glass)
Anthropogenic Amorphous Vitreous Silicate
Hardness: 5.5-6.5. Color: Dark green to black with grey porous base. Luster: Vitreous (glassy) on fresh breaks. Crystal structure: Amorphous. Specific gravity: 2.5-3.0. Features vesicles (gas bubbles) and conchoidal fracturing.
- Hardness
- 5
- Color
- Dark green to black with grey porous base
- Luster
- Vitreous (glassy) on fresh breaks
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Physical properties
Hardness: 5.5-6.5. Color: Dark green to black with grey porous base. Luster: Vitreous (glassy) on fresh breaks. Crystal structure: Amorphous. Specific gravity: 2.5-3.0. Features vesicles (gas bubbles) and conchoidal fracturing.
Formation & geological history
Formed as a byproduct of historical iron or steel smelting. When ore is melted, impurities fuse with flux to form a molten glass that cools into this rock-like material. These specimens are often over 100 years old.
Uses & applications
Historically used for road bedding and railroad ballast. Nowadays, it is a collector's curiosity or used in decorative landscaping and lapidary work.
Geological facts
Slag is often mistaken for obsidian or meteorites. Its green color usually indicates the presence of iron silicates. Many specimens found in the US and UK come from 19th-century blast furnaces.
Field identification & locations
Identify by the presence of round bubbles (vesicles), a glassy 'conchoidal' fracture, and a porous, rocky bottom layer. Common in old mining towns or near historical railway lines and ports.
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