mineraloid
Cullet Glass (Slag Glass)
Amorphous Silicon Dioxide (Industrial byproduct)
Hardness: 5.5-6.0 Mohs; Color: Deep cobalt blue; Luster: Vitreous (glassy); Structure: Amorphous (non-crystalline); Cleavage: Conchoidal (shell-like) fracture; Specific Gravity: 2.4-2.8
- Hardness
- 5
- Color
- Deep cobalt blue
- Luster
- Vitreous (glassy)
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Physical properties
Hardness: 5.5-6.0 Mohs; Color: Deep cobalt blue; Luster: Vitreous (glassy); Structure: Amorphous (non-crystalline); Cleavage: Conchoidal (shell-like) fracture; Specific Gravity: 2.4-2.8
Formation & geological history
Formed as a byproduct of glass manufacturing or smelting processes. Molten silica waste is dumped and cooled rapidly, preventing crystal formation. Often found in areas with historical industrial activity.
Uses & applications
Used in landscape design, terrazzo flooring, aquarium decor, and occasionally as 'fantasy' gemstones in costume jewelry or for tumbling.
Geological facts
While often mistaken for obsidian or rare minerals like azurite, this is an anthropogenic (man-made) material. Blue hues are usually caused by cobalt or copper additives in the original glass batch.
Field identification & locations
Identify by looking for air bubbles (vesicles), conchoidal fracture patterns, and intense, uniform colors not usually found in similar natural rocks. Common near old iron furnaces or glass factories.
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