Mineral-like byproduct

Slag

Anthropogenic glassy silicate

Hardness: 5-7 (Mohs); Color: Dark grey, black, or bluish-silver; Luster: Glassy to metallic; Structure: Amorphous/None; Cleavage: None (conchoidal fracture); Specific Gravity: Variable (2.5 - 4.0)

Hardness: 5-7 (Mohs)
Color: Dark grey, black, or bluish-silver
Luster: Glassy to metallic

Identified April 22, 2026More mineral-like byproduct →

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Physical properties

Hardness: 5-7 (Mohs); Color: Dark grey, black, or bluish-silver; Luster: Glassy to metallic; Structure: Amorphous/None; Cleavage: None (conchoidal fracture); Specific Gravity: Variable (2.5 - 4.0)

Formation & geological history

Formed as a byproduct of smelting ores (iron, steel, or copper) to separate the desired metal from the raw ore. While not naturally occurring, it is often found in the environment near historical industrial sites.

Uses & applications

Used in construction as road base, railroad ballast, roofing granules, and as an ingredient in cement. Historical slag is primarily of interest to amateur rockhounds.

Geological facts

Slag is often confused with meteorites or obsidian. The presence of 'vesicles' (small gas bubbles) and its glassy, melted appearance are key indicators of its man-made origin.

Field identification & locations

Identify in the field by looking for a 'bubbly' or 'pocked' surface, high weight relative to size (if metallic), and lack of organized crystal structure. Often found near old railroads or metal foundries.