Anthropogenic / Non-Natural

Slag (Industrial Glassy Byproduct)

Ferro-silicate Slag

Hardness: 5-7 (Mohs scale), Color: Black or dark grey with metallic or glassy areas, Luster: Vitreous to sub-metallic, Structure: Amorphous/Vesicular (contains bubbles/voids), Specific Gravity: Variable but often high due to iron content.

Identified April 12, 2026More anthropogenic / non-natural →

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Physical properties

Formation & geological history

Slag is a byproduct of metal smelting, particularly iron or steel production. It forms when limestone or other flux materials react with impurities in the ore and are separated from the molten metal. This specimen likely dates from the industrial era (late 19th to mid-20th century).

Uses & applications

Used historically as railroad ballast, road base material, or even in the manufacturing of concrete and mineral wool insulation. Smaller, colorful glass-like slag is sometimes used as decorative 'garden stones'.

Geological facts

Slag is often mistaken for meteorites or volcanic rocks like basalt or obsidian because of its dark color, bubbly texture (vesicles), and occasional magnetic properties.

Field identification & locations

Identify in the field by looking for 'bubbles' on both the surface and interior, flow lines comparable to melted glass, and its presence near former industrial sites or railroad tracks. It is usually much heavier than it looks if it has high metal content.

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