mineral

Slag Glass (Cullet Glass)

Amorphous Silica (Industrial byproduct)

Hardness: 5.5-6.5; Color: Vibrant blue to light blue with swirls; Luster: Vitreous (glassy); Crystal Structure: Amorphous (non-crystalline); Conchoidal fracture (curved, shell-like breaks).

Hardness: 5
Color: Vibrant blue to light blue with swirls
Luster: Vitreous (glassy)

Identified April 4, 2026More mineral →

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Physical properties

Hardness: 5.5-6.5; Color: Vibrant blue to light blue with swirls; Luster: Vitreous (glassy); Crystal Structure: Amorphous (non-crystalline); Conchoidal fracture (curved, shell-like breaks).

Formation & geological history

Slag glass is a byproduct of metal smelting or glass manufacturing processes. It forms when molten waste material cools quickly. While not a naturally occurring geologic formation, it is often found in areas with historical industrial activity.

Uses & applications

Primarily used for decorative landscaping, aquarium decor, glass floral arrangements, and as a low-cost lapidary material for hobbyists.

Geological facts

Many 'blue rocks' found near old railroad tracks or factories are actually slag. It often contains gas bubbles and swirled patterns (flow lines) that distinguish it from natural blue minerals like celestite or blue quartz.

Field identification & locations

Identify by looking for conchoidal fractures, air bubbles (vesicles), and a lack of organized crystal structure. Found near old steel mills, iron furnaces, or glass factories.

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