Slag Glass

Physical Properties

Hardness: 5.5-6.5 (Mohs scale); Color: Deep blue to teal; Luster: Vitreous (glassy); Crystal structure: Amorphous (non-crystalline); Cleavage: None (exhibits prominent conchoidal/curved fracture); Specific gravity: 2.4-2.8

Formation & Geological History

Formed as a byproduct of smelting ores (iron, steel, or glass production). When impurities and fluxes melt and then cool rapidly, they solidify into this glassy material. Generally dates from the 19th to early 20th century in industrial areas.

Uses & Applications

Historically used as landfill or road ballast; today used in decorative landscaping, glass art, tumbled for jewelry (imitation gems), and as a collector's item.

Geological Facts

While it looks like volcanic obsidian, the vibrant blue color is a result of chemical additives or specific mineral impurities in the industrial process. It is often found near historic railway lines or abandoned iron furnaces.

Field Identification & Locations

Identify in the field by its intense glass-like luster, sharp edges, air bubbles (vesicles), and conchoidal fractures. Commonly found in areas with an industrial history, particularly the Great Lakes region or the Rust Belt in the USA.

Identified on: 4/20/2026

Mode: Standard