Mineraloid / Industrial Waste
Slag Glass
Amorphous Silica Glass (with metal oxide inclusions)
Hardness: 5-6 (Mohs); Color: Vivid blue, teal, or green; Luster: Vitreous to dull; Structure: Amorphous (non-crystalline); Vesicular or bubbly texture common; Cleavage: None (conchoidal fracture).
- Hardness
- 5-6 (Mohs)
- Color
- Vivid blue, teal, or green
- Luster
- Vitreous to dull
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Physical properties
Hardness: 5-6 (Mohs); Color: Vivid blue, teal, or green; Luster: Vitreous to dull; Structure: Amorphous (non-crystalline); Vesicular or bubbly texture common; Cleavage: None (conchoidal fracture).
Formation & geological history
Formed as a byproduct of historical iron or copper smelting processes. Molten waste cooled rapidly on the surface or in pits, often near historical industrial sites such as New Jersey, Michigan, or Pennsylvania.
Uses & applications
Historical slag is often used for crafts, landscaping, and by 'glass hunters' as a collectible curiosity. Occasionally used in low-cost jewelry or as a decorative aquarium stone.
Geological facts
While not a natural 'rock' in the traditional sense, slag glass is a popular geological find. Different colors signify different mineral impurities: blue and green often indicate the presence of copper or iron oxides in the smelting mix.
Field identification & locations
Identify by looking for spherical bubbles (vesicles), a glassy or waxy shine, and unnatural colors like bright teal or electric blue. Often found in riverbeds near old furnace sites or former railroad beds.
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