mineraloid
Cobalt Glass Slug (Cullet)
Man-made Silicate Glass with Cobalt Oxide (CoO)
Hardness: 5.5-6 (Mohs); Color: Deep cobalt blue; Luster: Vitreous/Glassy; Structure: Amorphous (non-crystalline); Cleavage: Conchoidal fracture; Specific gravity: ~2.5
- Hardness
- 5
- Color
- Deep cobalt blue
- Luster
- Vitreous/Glassy
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Physical properties
Hardness: 5.5-6 (Mohs); Color: Deep cobalt blue; Luster: Vitreous/Glassy; Structure: Amorphous (non-crystalline); Cleavage: Conchoidal fracture; Specific gravity: ~2.5
Formation & geological history
Formed as a byproduct of industrial glass manufacturing or discarded decorative glass. It is an amorphous solid created by heating silica sand, soda ash, and limestone with cobalt additives.
Uses & applications
Used in landscaping, glass recycling, aquarium decoration, and sometimes lapidary work for inexpensive jewelry or art.
Geological facts
While it looks like a rare gemstone such as sapphire, its conchoidal fracture patterns and lack of crystal structure confirm it is glass. Cobalt glass has been used for centuries to create a distinctive deep blue pigment.
Field identification & locations
Identify by looking for air bubbles (vacuoles), swirling patterns, and sharp shell-like (conchoidal) edges. It is commonly found near old glass factories or dumped in waterways.
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