Iron Slag

Physical Properties

Hardness (Mohs scale 6-7), dark brown to black color, glassy to metallic luster, amorphous or microcrystalline structure, often contains vesicles (gas bubbles), specific gravity varies depending on iron content.

Formation & Geological History

Formed as a byproduct of smelting iron ore in a blast furnace. Modern slag is relatively recent (18th-20th century), occurring when impurities are separated from molten metal.

Uses & Applications

Used historically as railroad ballast, in road construction, and as an additive in cement. Rarely used in low-end decorative crafts.

Geological Facts

Slag is often mistaken for meteorites due to its dark color, weight, and occasional magnetic properties. It is a record of human industrial history in a specific region.

Field Identification & Locations

Identify by presence of vesicles (bubbles), flow textures, or a glassy fracture. Often found near old railroads, canal beds, or historical iron mill sites. Use a magnet to check for residual iron.

Identified on: 5/7/2026

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