Slag Glass

Physical Properties

Hardness: 5.5-6.5 (Mohs scale); Color: Black with green/blue banding; Luster: Vitreous/Glassy; Structure: Amorphous (non-crystalline); Fracture: Conchoidal; Bubbles: Presence of gas vesicles.

Formation & Geological History

Formed as a byproduct of smelting ores (mostly iron or copper) where impurities are separated from the metal. The molten waste cools rapidly into a glass-like solid. Most common specimens are from the 19th and early 20th centuries.

Uses & Applications

Historically used for railroad ballast, road construction, and shoreline protection. Modern uses include artistic landscaping, glass cullet for recycling, and 'Leeland Blue' or 'Frankfort Green' jewelry making.

Geological Facts

While not a mineral by geological definition because it is man-made, it mimics volcanic obsidian. The distinct banding and bubbles are hallmarks of its industrial origin rather than natural processes.

Field Identification & Locations

Identify by looking for spherical bubbles (vesicles), swirl patterns (flow lines), and a glass-like conchoidal fracture. Often found near old railroad tracks, iron foundry sites, or Great Lakes beaches.

Identified on: 4/15/2026

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