Green Slag Glass

Physical Properties

Hardness: 5.5-6.5 (Mohs scale), Color: Dark forest green to teal, Luster: Vitreous to glassy, Structure: Amorphous (non-crystalline), Cleavage: None (conchoidal fracture), Specific Gravity: 2.4-2.8

Formation & Geological History

Slag is a byproduct of historical iron or glass smelting processes. It forms when molten waste material cools rapidly, often resulting in a glassy or vesicular (bubbly) texture. Most found today dates from the 19th or early 20th centuries.

Uses & Applications

Historically a waste product, it is now used for garden landscaping, decorative rockery, aquarium stones, and occasionally tumbled for jewelry or 'sea glass' collecting.

Geological Facts

Slag glass is often mistaken for rare volcanic stones like obsidian or moldavite, but its presence near old industrial sites or railways is a major giveaway. The bubbles inside are trapped gases from the smelting furnace.

Field Identification & Locations

Identify by looking for conchoidal (shell-like) fractures, trapped air bubbles (vesicles), and a lack of crystal structure. It is commonly found near old steel mills, iron furnaces, or railroad beds where it was used as ballast.

Identified on: 4/23/2026

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