Mineraloid (Industrial byproduct)

Green Slag

Anthropogenic Silicate Glass

Hardness: 5-6 (Mohs); Color: Deep forest green; Luster: Vitreous (glassy); Crystal structure: Amorphous; Cleavage: None (conchoidal fracture); Specific gravity: 2.4-2.8. Often contains spherical gas bubbles (vesicles).

Hardness: 5-6 (Mohs)
Color: Deep forest green
Luster: Vitreous (glassy)

Identified January 15, 2026More mineraloid (industrial byproduct) →

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Physical properties

Formation & geological history

Formed as a byproduct of historical smelting processes (iron, copper, or glass making). Geological age is modern to post-Industrial Revolution. It cooling quickly from a molten state prevents crystal growth, resulting in a glass form.

Uses & applications

Historically used as railroad ballast, road aggregate, and sometimes as a low-cost gemstone (often marketed as 'monatomic andara' or 'swirl glass'). Today it is primarily a collector's item for those interested in industrial archaeology.

Geological facts

Many specimens, like this one, are mistaken for obsidian or moldavite. However, the presence of perfectly round gas bubbles and it being found in areas with a history of smelting are distinctive indicators of its man-made origin.

Field identification & locations

Identify in the field by looking for conchoidal fractures and internal bubbles. Common in locations near old blast furnaces or iron works (e.g., Leland Blue in Michigan). Collectors should check for high transparency and unique swirls of color.

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