Industrial byproduct (often mistaken for obsidian or mineral)

Green Slag Glass

Amorphous Silica Glass (Anthropogenic)

Hardness: 5.5-6 on Mohs scale. Color: Teal to sea-green, often with white or grey crusting. Luster: Vitreous (glassy) when broken, dull when weathered. Structure: Amorphous (non-crystalline). Cleavage: None, exhibits conchoidal fracture. Often contains spherical gas bubbles (vesicles).

Hardness: 5
Color: Teal to sea-green, often with white or grey crusting
Luster: Vitreous (glassy) when broken, dull when weathered

Identified March 24, 2026More industrial byproduct (often mistaken for obsidian or mineral) →

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Physical properties

Formation & geological history

Formed as a byproduct of smelting iron, copper, or other ores in blast furnaces. Impurities like limestone, silica, and mineral oxides are removed from metal and cool into glass. Most specimens found in the US date to the 19th or early 20th centuries.

Uses & applications

Historically used as railroad ballast, road fill, or discarded as waste. Currently popular among rockhounds and lapidaries for tumble polishing, aquarium decor, and folk art jewelry.

Geological facts

Also known as 'Leland Blue' or 'Frankfurt Green' depending on the region and the specific smelter it came from. Its color is often caused by trace amounts of chemical additives like copper or chromium used in the industrial process.

Field identification & locations

Identify by looking for spherical bubbles (which do not occur in natural obsidian) and conchoidal fractures. Commonly found near historical iron working sites, old railway beds, and Great Lakes beaches. It is a sign of local industrial history.

Other industrial byproduct (often mistaken for obsidian or mineral) specimens

See all

Slag (Industrial By-product)

Anthropogenic Vitreous Silica/Iron Silicate

mineral

Man-made Slag (Industrial Byproduct)

Anthropogenic Glassy Residue

not a natural rock

Cullet Glass (Slag Glass)

Amorphous Silicon Dioxide (Industrial Byproduct)

mineraloid

Slag (Industrial Glass)

Artificial Vitreous Silicate

mineraloid (anthropogenic)

Slag (Industrial Glassy Byproduct)

Anthropogenic silicate byproduct (Slag)

Mineral-like byproduct (often mistaken for igneous rock or meteorite)

Ferrosilicon Slag (Industrial Vitrified Glass)

Vitreous Silicate Residue

Antropogenic (Man-made industrial byproduct)