Mineraloid / Synthetic Glass
Slag Glass (Cullet)
Amorphous Silica (man-made), primarily SiO2 with trace metal oxides
Hardness: 5.5-6.0 on Mohs scale; Color: Cerulean blue with dark grey/black swirls; Luster: Vitreous (glassy); Structure: Amorphous (non-crystalline); Fracture: Conchoidal (shell-like curves).
- Hardness
- 5
- Color
- Cerulean blue with dark grey/black swirls
- Luster
- Vitreous (glassy)
Identified More mineraloid / synthetic glass →
Identify your own rocks.
Get a report just like this from any photo, free.
Physical properties
Hardness: 5.5-6.0 on Mohs scale; Color: Cerulean blue with dark grey/black swirls; Luster: Vitreous (glassy); Structure: Amorphous (non-crystalline); Fracture: Conchoidal (shell-like curves).
Formation & geological history
Formed as a byproduct of industrial processes (glassmaking or smelting) or specifically manufactured as decorative 'slag' art. Most specimens found today date from the late 19th to mid-20th century.
Uses & applications
Used for decorative garden stones, fish tanks, paperweights, or as high-end collector marbles (swirled glass). Historically, it was a waste product repurposed for landscape filler.
Geological facts
Slag glass is often mistaken for natural stones like obsidian or blue chalcedony, but the presence of internal air bubbles and swirl patterns are key identifiers of its manufactured origin.
Field identification & locations
Identify by looking for swirling colors that don't follow natural crystal growth and the presence of tiny spherical air bubbles. Often found near old factory sites, riverbeds near glassworks, or sold as decorative spheres.
More like this
Other mineraloid / synthetic glass specimens
Cobalt Glass Slug (Man-made Glass)
Amorphous Silica with Cobalt Oxide (SiO2 + CoO)
mineraloid
Opalite
Synthetic Glass (Amorphized Silicon Dioxide)
mineraloid
Opalite (Synthetic Moonstone/Opal)
Synthetic Silica Glass
mineraloid (synthetic)
Opalite
Synthetic Opalised Glass
mineraloid (synthetic)
Opalite
Synthetic Glass (Hydrated Crystalline Silicon Dioxide simulation)
mineral
Man-made Glass (Tumbled/Shaped)
Amorphous Silicon Dioxide (SiO2) with metal oxide colorants
Mineraloid (synthetic)