Anthropogenic (Man-made)

Blue Slag Glass

Man-made Silicon Dioxide (Amorphous)

AI-generated identification · may be incorrect

Hardness: 5.5-6.5 (Mohs scale), Color: Cobalt to cerulean blue, Luster: Vitreous, Crystal Structure: Amorphous (non-crystalline), Cleavage: None (conchoidal fracture), Specific Gravity: 2.4-2.8

Hardness: 5

Identified June 28, 2026More anthropogenic (man-made) →

Explore Blue Slag Glass in the encyclopedia →

Identify your own rocks.

Get a report just like this from any photo, free.

App Store Google Play

Physical properties

Hardness: 5.5-6.5 (Mohs scale), Color: Cobalt to cerulean blue, Luster: Vitreous, Crystal Structure: Amorphous (non-crystalline), Cleavage: None (conchoidal fracture), Specific Gravity: 2.4-2.8

Formation & geological history

Formed as a byproduct of industrial smelting processes or glass manufacturing. When waste material cools rapidly, it solidifies into a glass-like state rather than a crystalline rock. Although not a mineral, it is often found in geological contexts.

Uses & applications

Historically discarded as waste, but now popular for aquarium decor, landscaping, metaphysical collections, and lapidary work (cabochons).

Geological facts

Slag is often mistaken for rare gemstones like blue obsidian or blue amber. Its vibrant blue hue usually comes from the presence of cobalt or copper ions during the smelting process.

Field identification & locations

Identify by looking for conchoidal (shell-like) fractures and the presence of small air bubbles or 'swirl' marks. Commonly found near old iron or glass foundry sites and near historical railroad beds used for ballast.