industrial byproduct

Slag Glass

Glassy Silicate Residue

Hardness: 5.5-6 (Mohs); Color: Cobalt blue, translucent; Luster: Vitreous/glassy; Structure: Amorphous; Cleavage: None (conchoidal fracture); Specific Gravity: 2.4-2.8

Hardness: 5
Color: Cobalt blue, translucent
Luster: Vitreous/glassy

Identified May 10, 2026More industrial byproduct →

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Physical properties

Hardness: 5.5-6 (Mohs); Color: Cobalt blue, translucent; Luster: Vitreous/glassy; Structure: Amorphous; Cleavage: None (conchoidal fracture); Specific Gravity: 2.4-2.8

Formation & geological history

Slag glass is a byproduct of the metal smelting process, specifically from iron or copper ore refinement. It forms when the molten waste materials (silica, calcium, alumina) cool rapidly, often appearing at historic industrial sites or old railroad beds.

Uses & applications

Historically discarded as waste, it is now used in decorative landscaping, aquarium design, glassmaking, and occasionally shaped into jewelry or 'slag glass' collectibles.

Geological facts

The intense blue color usually results from cobalt additives or specific chemical reactions during the smelting process. It is often mistaken for rare volcanic obsidian or gemstones like sapphire.

Field identification & locations

Identify by the conchoidal (shell-like) fracture, the presence of small gas bubbles (vesicles), and its highly glassy, translucent appearance. It is commonly found near old factories or along tracks where it was used as ballast.