Mineraloid / Industrial Byproduct

Red Slag Glass

Amorphous Silica (Industrial byproduct with metallic oxide dopants)

Hardness: 5.5-6.0 Mohs; Color: Vibrant cherry or brick red; Luster: Vitreous/Glassy; Structure: Amorphous (non-crystalline); Fracture: Conchoidal (shell-like curves); Specific Gravity: 2.4-2.8

Hardness: 5
Color: Vibrant cherry or brick red
Luster: Vitreous/Glassy

Identified March 15, 2026More mineraloid / industrial byproduct →

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Physical properties

Hardness: 5.5-6.0 Mohs; Color: Vibrant cherry or brick red; Luster: Vitreous/Glassy; Structure: Amorphous (non-crystalline); Fracture: Conchoidal (shell-like curves); Specific Gravity: 2.4-2.8

Formation & geological history

Slag glass is a byproduct of the smelting process (iron or glass manufacturing). It forms when molten silica cools rapidly on the surface of slag piles. While it mimics natural obsidian, it is man-made and typically dates from the 19th century to modern times.

Uses & applications

Commonly used for landscaping rock, vintage glass collecting, or as decorative aquarium stone. Higher-end pieces are sometimes tumbled and used in wire-wrapped costume jewelry.

Geological facts

Red is one of the more sought-after colors in slag glass because it often requires the presence of gold or selenium in the mix to achieve the deep hue. Many collectors initially mistake it for 'Mahogany Obsidian' or 'Dragon Glass'.

Field identification & locations

Identify in the field by looking for tiny air bubbles (vesicles) trapped inside and the absence of any natural mineral inclusions. It is most frequently found in areas with a history of heavy industry or iron foundries.

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