Cullet (Slag Glass)

Physical Properties

Hardness: 5.5-6.5; Color: Light blue/Aqua; Luster: Vitreous (glassy); Crystal Structure: Amorphous (none); Cleavage: None (conchoidal fracture); Specific Gravity: 2.4-2.8

Formation & Geological History

Formed as a byproduct of industrial glass manufacturing or smelting processes. While it shares chemical properties with obsidian, it is man-made and can be found in sites associated with historical iron or glass production.

Uses & Applications

Predominantly used as decorative landscape stone, in aquariums, or as raw material to be remelted for new glass. Sometimes used in costume jewelry or as 'faux' gemstones.

Geological Facts

Many finders mistake light blue cullet for rare minerals like aquamarine or blue obsidian. Blue coloration is often achieved through the addition of copper or cobalt during the industrial heating process.

Field Identification & Locations

Identify by its perfect conchoidal (shell-like) fractures and presence of internal air bubbles (gas vesicles), which are rare in natural crystals. Often found near old railroad beds or industrial dump sites.

Identified on: 4/25/2026

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