mineral

Slag

Anthropogenic Iron Silicate Glass

Hardness: 6-7 on Mohs scale; Color: Black, dark grey, or metallic; Luster: Submetallic to vitreous; Structure: Amorphous (glassy); Cleavage: None/Conchoidal fracture; Specific gravity: 2.5-4.5 (variable).

Hardness: 6-7 on Mohs scale
Color: Black, dark grey, or metallic
Luster: Submetallic to vitreous

Identified May 2, 2026More mineral →

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Physical properties

Formation & geological history

Formed as a byproduct of smelting ores (such as iron or copper) where metals are separated from impurities. It consists of melted rock and fluxes that cool rapidly. These specimens are often modern or historical (industrial age).

Uses & applications

Used in railroad ballast, road construction, filtration media, and abrasive blasting. Collectors often keep glassier specimens for their unique swirl patterns.

Geological facts

Slag is often mistaken for meteorites due to its dark color, metallic sheen, and occasional bubbles (vesicles), which real meteorites almost never have. It represents a record of human industrial history.

Field identification & locations

Identify by looking for vesicles (gas bubbles), flow lines on the surface, and an glassy fracture. It is commonly found near old railroads, canal banks, and former iron smelting sites.