Slag

Physical Properties

Hardness: 5-6 (Mohs); Color: Dark brown, black, or grey; Luster: Glassy to metallic; Crystal Structure: Amorphous/glassy; Cleavage: None (conchoidal fracture); Specific Gravity: 2.5-4.5 depending on metal content.

Formation & Geological History

Formed as a byproduct of historical or industrial smelting. When ore is heated, impurities separate into a molten silicate liquid that cools rapidly into a glass-like or stony material. Some pieces may be hundreds of years old from colonial forges.

Uses & Applications

Historically a waste product; modernly used as road ballast, railway track bedding, and in certain types of concrete or insulation (rock wool). Collectors value it for historical context or aesthetic 'desert glass' appearance.

Geological Facts

Slag is often mistaken for meteorites due to its dark, melted appearance and occasional magnetic properties, but the presence of vesicles (air bubbles) almost always identifies it as terrestrial slag.

Field Identification & Locations

Identify by looking for spherical air bubbles (vesicles), glassy texture, and high density. Commonly found near old railroad tracks, historical industrial sites, and iron bogs.

Identified on: 4/28/2026

Mode: Standard