mineral

Slag Glass

Anthropogenic Amorphous Silicon Dioxide

Hardness: 5.5-6.5 (Mohs scale); Color: Variable, frequently green, blue, or violet; Luster: Vitreous to glassy; Structure: Amorphous (non-crystalline); Fracture: Distinctive conchoidal (curved/shell-like) fractures; Specific Gravity: 2.4-2.8

Hardness: 5
Color: Variable, frequently green, blue, or violet
Luster: Vitreous to glassy

Identified February 12, 2026More mineral →

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Physical properties

Formation & geological history

Formed as a byproduct of historical smelting processes (iron, steel, or copper) or glass manufacturing. It is not geological in origin but is a man-made waste product that has cooled and hardened over time, often found in areas with past industrial activity.

Uses & applications

Historically a waste product, it is now used for decorative landscaping, jewelry (lapidary work), aquarium gravel, and as a collectible 'curiosity'. Dense slag was sometimes used as road ballast or railroad sub-base.

Geological facts

Many beach-goers and hikers mistake slag for rare minerals like obsidian or emerald. It often contains trapped air bubbles (vesicles), which is a key indicator that it is man-made glass rather than a natural volcanic stone like obsidian.

Field identification & locations

Identify by looking for glass-like transparency on edges, conchoidal fracture patterns, and tiny internal gas bubbles. Common near historical furnace sites, rivers near old industrial towns, and Great Lakes beaches. Collectors look for 'Leland Blue' or 'Frankfort Green' varieties.

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