Identification guide · Materials
How to Identify Glass
Glass is usually green, brown, and clear 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 Slag Glass, Obsidian, and Opalite; the tests below separate them.
Glass identification checklist
| Color | Green, Brown, Clear, Blue, White, Frosted | 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 Glass
Glass check. 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.
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. Glass vs Slag Glass →
Both sit near Mohs 5.5, so a scratch test alone will not separate them. Lean on luster, streak and heft instead. 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. Glass vs Opalite →
Quartz is harder (Mohs 7) than Glass (Mohs 5.5), so Quartz will scratch Glass and not the other way round. It is about as hard as window glass: a knife barely marks it, while a steel file (Mohs 6.5) will. Glass vs Quartz →
Field tips
- Frosted surfaces with smooth rounded edges point to surf-tumbled glass.
- Look for tiny bubbles, flow lines, or embossed lettering fragments from bottles.
- Glass scratches with a steel knife more easily than quartz and shows perfect conchoidal fracture.
Glass identification FAQ
How can you tell if a rock is Glass?
Check the color (green, brown, clear, and blue), 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 Glass?
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 Glass and Slag Glass?
Glass (Mohs 5.5, vitreous luster) and Slag Glass (Mohs 5.5, vitreous luster) look alike, but hardness, streak and heft usually separate them. See the full Glass vs Slag Glass comparison for a side-by-side table.
Is Glass worth anything?
Entry-level Glass specimens sell for around $1, typical collector pieces about $5, and premium examples $40 or more depending on size, quality and locality.
What color is Glass?
Glass is most often green, brown, clear, and blue. Color varies with impurities and locality, so confirm with hardness and luster rather than color alone.
