igneous (man-made/industrial)
Blue Slag Glass
Ferro-silicate glass (Slag)
Hardness: 5.5-6.5 (Mohs scale), Color: Deep teal/blue, Luster: Vitreous/Glassy, Crystal Structure: Amorphous (non-crystalline), Cleavage: None (conchoidal fracture), Specific Gravity: 2.5-3.0
- Hardness
- 5
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Physical properties
Hardness: 5.5-6.5 (Mohs scale), Color: Deep teal/blue, Luster: Vitreous/Glassy, Crystal Structure: Amorphous (non-crystalline), Cleavage: None (conchoidal fracture), Specific Gravity: 2.5-3.0
Formation & geological history
Formed as a byproduct of iron or copper smelting where limestone and silica are exposed to high heat. While not 'geological' in age, many specimens date to the 19th and early 20th centuries during the Industrial Revolution.
Uses & applications
Used historically as ballast, road filler, and rail bed gravel; currently popular for lapidary work, landscaping, and as a collector's item for rockhounds.
Geological facts
Slag is often mistaken for obsidian or rare gemstones like sapphire. The striking blue color is often caused by trace amounts of cobalt, copper, or manganese in the smelting process.
Field identification & locations
Identify in the field by looking for air bubbles (vesicles), conchoidal fracture (curved, glass-like breaks), and proximity to old industrial/railroad sites. It lacks the crystal structure of natural minerals.
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