Man-made byproduct (Non-geological)
Slag Glass (Cullet)
Amorphous Silica / Glass residue
Hardness: 5.5-6.5 (Mohs scale); Color: Vibrant lime green with yellow swirled bands; Luster: Vitreous (glassy); Structure: Amorphous (non-crystalline); Specific Gravity: 2.4-2.8
- Hardness
- 5
- Color
- Vibrant lime green with yellow swirled bands
- Luster
- Vitreous (glassy)
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Physical properties
Hardness: 5.5-6.5 (Mohs scale); Color: Vibrant lime green with yellow swirled bands; Luster: Vitreous (glassy); Structure: Amorphous (non-crystalline); Specific Gravity: 2.4-2.8
Formation & geological history
Formed as a byproduct of industrial processes such as glass manufacturing or iron smelting (where it is called blast furnace slag). It is created when molten waste cools rapidly, trapping swirling colors or air bubbles. It is not an naturally occurring rock, though it is often found in older industrial areas.
Uses & applications
Commonly used as decorative garden stones, aquarium gravel, or tumble-polished for 'healing crystal' shops. Unique pieces are sometimes used in lapidary work to create cabochons for jewelry.
Geological facts
While it looks like a mineral, the presence of perfectly round internal air bubbles and the 'swirl' pattern of color intensity are dead giveaways that it is glass. Slag glass from the mid-20th century is highly collectible, especially from famous glassworks like Akro Agate or Fenton.
Field identification & locations
Identify by looking for conchoidal (shell-like) fractures and air bubbles. Unlike natural minerals, the colors in slag are often unnaturally bright or neon. It is most commonly found near old glass factories, riverbeds draining from industrial sites, or in landscaping decor.
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