Mineraloid (Industrial byproduct)

Slag (Industrial Glass/Metal Waste)

Artificial silicate/ferro-silicate byproduct

Hardness: 5-7 (Mohs scale); Color: Dark grey, brownish-black, or variegated; Luster: Glassy (vitreous) to dull/earthy; Structure: Amorphous (non-crystalline); Features: Vesicular (bubbly) texture with occasional flow lines and sharp, glass-like edges where broken.

Hardness: 5-7 (Mohs scale)
Color: Dark grey, brownish-black, or variegated
Luster: Glassy (vitreous) to dull/earthy

Identified March 14, 2026More mineraloid (industrial byproduct) →

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Physical properties

Formation & geological history

Formed as a byproduct of smelting ores (iron, steel, or copper) where impurities are separated from the molten metal. The 'bubbly' texture occurs when gases are trapped during rapid cooling. This specimen likely dates from the 19th or 20th century industrial era.

Uses & applications

Historically used as road ballast, railroad fill, and aggregate for concrete. Today, slag is recycled in cement production or used as an abrasive in sandblasting.

Geological facts

Slag is frequently mistaken for meteorites (often called 'meteor-wrongs') because it can be heavy, dark, and possess a melted-looking surface. Unlike real meteorites, slag often contains air bubbles (vesicles), which real space rocks almost never have.

Field identification & locations

Identify in the field by looking for 'glassy' surfaces combined with a porous, sponge-like or bubbly appearance. It is commonly found near old railways, abandoned smelting sites, or used as landscaping rock. Collectors use the 'streak test' (it usually leaves a gray or colorless streak) to distinguish it from metallic ores.

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