Identification guide · Materials
How to Identify Slag Glass
Slag Glass is usually blue, green, and black with a vitreous luster and is translucent. It has a Mohs hardness of 5.5. It is about as hard as window glass: a knife barely marks it, while a steel file (Mohs 6.5) will. Its streak is white to colorless. It is most often confused with Glass, Opal, and Obsidian; the tests below separate them.
Slag Glass identification checklist
| Color | Blue, Green, Black, Brown, White, Swirled | Look at a clean, dry surface in daylight. Color zoning, banding or spots are as diagnostic as the hue. |
|---|---|---|
| Luster | Vitreous | Fresh surfaces shine like glass; a dull surface usually means weathering, so chip or wet a corner. |
| Transparency | Translucent | Hold a thin edge up to a bright light and note whether light passes through. |
| Mohs hardness | 5.5 | It is about as hard as window glass: a knife barely marks it, while a steel file (Mohs 6.5) will. |
| Streak | White to colorless | Drag a corner across unglazed porcelain (the back of a bathroom tile) and check the powder color. |
| Cleavage | None | |
| Fracture | Conchoidal | Broken surfaces show smooth, curved, shell-like scoops. |
| Specific gravity | 2.5 | Ordinary rock-like heft (specific gravity about 2.5), similar to quartz. |
| Crystal system | Amorphous |
Extra tests for Slag Glass
Glass check. Slag Glass shares a glassy luster and conchoidal fracture with man-made glass. Glass sits near Mohs 5.5 and often contains round gas bubbles, so a hardness test and a hand lens separate them.
Slag Glass look-alikes and how to tell them apart
Both sit near Mohs 5.5, so a scratch test alone will not separate them. Lean on luster, streak and heft instead. Slag Glass vs Glass →
Both sit near Mohs 5.5, so a scratch test alone will not separate them. Lean on luster, streak and heft instead. Slag Glass vs Opal →
Both sit near Mohs 5.5, so a scratch test alone will not separate them. Lean on luster, streak and heft instead. Slag Glass vs Obsidian →
Both sit near Mohs 5.5, so a scratch test alone will not separate them. Lean on luster, streak and heft instead. Slag Glass vs Opalite →
Field tips
- Look for bubbles, flow bands, glassy fracture, and unnatural color swirls.
- Industrial context or sharp glassy chips strongly supports slag glass.
- Compare against natural Obsidian, Chert, and Agate before assigning a geological name.
Slag Glass identification FAQ
How can you tell if a rock is Slag Glass?
Check the color (blue, green, black, and brown), the vitreous luster, a Mohs hardness of 5.5 and a white to colorless streak. Photographing it with the StoneSnap app gives a match in seconds.
What is the hardness of Slag Glass?
Slag Glass has a Mohs hardness of 5.5. It is about as hard as window glass: a knife barely marks it, while a steel file (Mohs 6.5) will.
What is the difference between Slag Glass and Glass?
Slag Glass (Mohs 5.5, vitreous luster) and Glass (Mohs 5.5, vitreous luster) look alike, but hardness, streak and heft usually separate them. See the full Slag Glass vs Glass comparison for a side-by-side table.
Is Slag Glass worth anything?
Entry-level Slag Glass specimens sell for around $1, typical collector pieces about $10, and premium examples $80 or more depending on size, quality and locality.
What color is Slag Glass?
Slag Glass is most often blue, green, black, and brown. Color varies with impurities and locality, so confirm with hardness and luster rather than color alone.
