man-made glass

Slag Glass

SiO2 (Silicon Dioxide) with impurities

Hardness: 5.5-6 on Mohs scale. Color: Vivid blue (often due to cobalt). Luster: Vitreous (glassy). Crystal structure: Amorphous (non-crystalline). Cleavage: None (conchoidal fracture). Specific Gravity: Varies, typically around 2.5.

Hardness: 5
Color: Vivid blue (often due to cobalt)
Luster: Vitreous (glassy)

Identified July 21, 2026More man-made glass →

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Physical properties

Formation & geological history

Slag glass is a byproduct of metal smelting processes or glass manufacturing. It is a man-made material, formed when impurities and fluxing agents melt together and form a glassy substance that separates from the metal. The blue color often indicates the presence of cobalt.

Uses & applications

Historically a waste product, it is now often collected for its aesthetic appeal, used in landscaping, as decorative aggregate, or sometimes cut and polished for inexpensive jewelry or crafts.

Geological facts

Also known as 'cullet', slag glass can be found in large chunks near old smelting or glass production sites. Its bright colors and glassy texture often lead people to mistake it for rare gemstones.

Field identification & locations

Identify by its glassy luster, conchoidal fracture (curved breakages like glass), lack of crystal structure, and presence of bubbles (vesicles) within the material. Commonly found near old industrial sites or railroad tracks.

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