mineraloid / industrial byproduct

Cullet Glass (sometimes mistaken for Blue Obsidian or Slag)

Amorphous Silicon Dioxide (SiO2) with chemical additives (e.g., Cobalt for blue color)

Hardness: 5.5—6.5 (Mohs scale); Color: Deep cobalt blue; Luster: Vitreous (glassy); Structure: Amorphous (non-crystalline); Cleavage: None (conchoidal fracture); Specific Gravity: 2.4—2.8.

Hardness: 5
Color: Deep cobalt blue
Luster: Vitreous (glassy)

Identified April 8, 2026More mineraloid / industrial byproduct →

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Physical properties

Hardness: 5.5—6.5 (Mohs scale); Color: Deep cobalt blue; Luster: Vitreous (glassy); Structure: Amorphous (non-crystalline); Cleavage: None (conchoidal fracture); Specific Gravity: 2.4—2.8.

Formation & geological history

Formed as a byproduct of industrial glass manufacturing or as left-over 'waste' glass cooled in large chunks. While it mimics the cooling process of volcanic obsidian, its vibrant blue color is almost always a result of human-added pigments.

Uses & applications

Used in landscaping, decorative stone masonry, aquariums, spiritual/metaphysical collecting, and sometimes faceted as a budget gemstone.

Geological facts

Many specimens sold as 'Blue Obsidian' are actually man-made cullet glass. True blue volcanic obsidian is extremely rare and usually presents as a faint grayish-blue, rather than the vivid cobalt seen in industrial glass.

Field identification & locations

Identify by looking for small trapped air bubbles (gas vesicles) and a lack of natural inclusions. In the field, look for 'outcrops' near old glass factories or iron smelting sites (slag glass).

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Slag (Industrial Glass Slag)

Amorphous Silicate Glass (often containing Al, Ca, Mg, and Ti oxides)

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Green Slag

Anthropogenic Silicate Glass

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Slag (Industrial Glass/Metal Waste)

Artificial silicate/ferro-silicate byproduct

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Cullet Glass (Slag Glass)

Amorphous Silicon Dioxide (Industrial Byproduct)

mineraloid