Slag (Industrial Glass)

Physical Properties

Hardness: 5.5-6.5 on Mohs scale; Color: Dark teal/green to black; Luster: Vitreous (glassy); Structure: Amorphous (non-crystalline); Cleavage: Conchoidal fracture; Specific gravity: 2.5-4.0 depending on metal content.

Formation & Geological History

Formed as a byproduct of smelting ores (such as iron or copper) during the 19th and early 20th centuries. The silicate-rich waste cools rapidly, forming a glass-like substance. While technically anthropogenic, it is often found in riverbeds and old industrial sites.

Uses & Applications

Historically used for railroad ballast, road construction, and occasionally in rough jewelry or as garden ornaments. Today, primarily a collector's item for those interested in industrial archaeology.

Geological Facts

Slag is often mistaken for obsidian or meteorites. Often referred to as 'Leland Blue' if found in Michigan or 'Bayate' in other regions. It frequently contains small air bubbles (vesicles) trapped during rapid cooling, which natural stones rarely have.

Field Identification & Locations

Identify in the field by checking for bubble-like cavities, extreme glassiness, and historical proximity to industrial sites or rail lines. Its vibrant teal color is usually a result of chemical additives not common in surrounding natural rocks.

Identified on: 4/14/2026

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