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

Identified May 15, 2026More mineraloid →

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Physical properties

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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