mineraloid
Cobalt Blue Slag Glass
Amorphous Silicon Dioxide (anthropogenic) with Cobalt additives
Hardness: 5.5-6.5 (Mohs scale), Color: Intense cobalt blue, Luster: Vitreous/glassy, Crystal Structure: Amorphous, Cleavage: None (conchoidal fracture), Specific Gravity: 2.4-2.8
- Hardness
- 5
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Physical properties
Hardness: 5.5-6.5 (Mohs scale), Color: Intense cobalt blue, Luster: Vitreous/glassy, Crystal Structure: Amorphous, Cleavage: None (conchoidal fracture), Specific Gravity: 2.4-2.8
Formation & geological history
Formed as a byproduct of historical iron or glass smelting processes. This material is essentially artificial volcanic glass (obsidian counterpart) produced during the industrial era when silicate impurities were removed from metal ores.
Uses & applications
Historically a waste product; modernly used for landscaping, aquarium decor, decorative jewelry, and as a collectible for 'slag hunters'.
Geological facts
Many beginning collectors mistake this for blue obsidian or rare sapphire, but true blue obsidian is extremely rare in nature and rarely this vibrant. The color comes from small amounts of cobalt oxide added during the smelting process.
Field identification & locations
Identify by looking for conchoidal (shell-like) fractures, bubbles (vesicles) trapped inside, and a lack of a regular crystal structure. It is most commonly found near old iron furnace sites or glass factory dump sites.
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