industrial byproduct
Slag Glass
Amorphous Silicate (Man-made)
Hardness: 5.5-6.5; Color: Deep teal/blue-green; Luster: Vitreous to dull; Crystal structure: Amorphous (glassy); Cleavage: None (conchoidal fracture); Specific gravity: 2.4-2.8
- Hardness
- 5
- Color
- Deep teal/blue-green
- Luster
- Vitreous to dull
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Physical properties
Hardness: 5.5-6.5; Color: Deep teal/blue-green; Luster: Vitreous to dull; Crystal structure: Amorphous (glassy); Cleavage: None (conchoidal fracture); Specific gravity: 2.4-2.8
Formation & geological history
Formed as a byproduct of iron or copper smelting processes when ore is heated to high temperatures; the impurities separate and cool rapidly into glass. Found in areas with historical industrial activity.
Uses & applications
Historically discarded as waste; currently used for landscaping, glass art, or collected as 'industrial fossils'. In some cases, it was used for road ballast.
Geological facts
Slag glass can be mistaken for obsidian or jasper, but its tell-tale signs are air bubbles and swirls of color from chemical impurities. It remains a popular find for beachcombers along the Great Lakes.
Field identification & locations
Identify by looking for conchoidal (shell-like) fracture patterns and tiny spherical gas bubbles trapped inside. Often found near old furnace or smelting sites.
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