Identification guide · Igneous Rocks
How to Identify Phonolite
Phonolite is usually gray, greenish gray, and brown with a dull luster and is opaque. It has a Mohs hardness of 6. It is about as hard as window glass: a knife barely marks it, while a steel file (Mohs 6.5) will. Its streak is light gray to white. It is most often confused with Basalt, Basaltic Andesite, and Trachyte; the tests below separate them.
Phonolite identification checklist
| Color | Gray, Greenish gray, Brown, Black, White | Look at a clean, dry surface in daylight. Color zoning, banding or spots are as diagnostic as the hue. |
|---|---|---|
| Luster | Dull | Compare a fresh surface with glass and metal; this luster is softer than either. |
| Transparency | Opaque | Hold a thin edge up to a bright light and note whether light passes through. |
| Mohs hardness | 6 | It is about as hard as window glass: a knife barely marks it, while a steel file (Mohs 6.5) will. |
| Streak | Light gray to white | Drag a corner across unglazed porcelain (the back of a bathroom tile) and check the powder color. |
| Cleavage | None to variable by phenocrysts | |
| Fracture | Uneven to conchoidal | Broken surfaces show smooth, curved, shell-like scoops. |
| Specific gravity | 2.6 | Ordinary rock-like heft (specific gravity about 2.6), similar to quartz. |
Phonolite look-alikes and how to tell them apart
Both sit near Mohs 6, so a scratch test alone will not separate them. Lean on luster, streak and heft instead. Phonolite vs Basalt →
Both sit near Mohs 6, so a scratch test alone will not separate them. Lean on luster, streak and heft instead. Phonolite vs Basaltic Andesite →
Both sit near Mohs 6, so a scratch test alone will not separate them. Lean on luster, streak and heft instead. Phonolite vs Trachyte →
Both sit near Mohs 6, so a scratch test alone will not separate them. Lean on luster, streak and heft instead.
Field tips
- Look for gray to greenish gray fine-grained volcanic texture.
- Phonolite can be hard to confirm from photos unless locality or mineral context supports it.
- It usually lacks obvious quartz.
- Do not confuse it with ordinary basalt or andesite without alkali-rock clues.
Phonolite identification FAQ
How can you tell if a rock is Phonolite?
Check the color (gray, greenish gray, brown, and black), the dull luster, a Mohs hardness of 6 and a light gray to white streak. Photographing it with the StoneSnap app gives a match in seconds.
What is the hardness of Phonolite?
Phonolite has a Mohs hardness of 6. 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 Phonolite and Basalt?
Phonolite (Mohs 6, dull luster) and Basalt (Mohs 6, dull luster) look alike, but hardness, streak and heft usually separate them. See the full Phonolite vs Basalt comparison for a side-by-side table.
Is Phonolite worth anything?
Entry-level Phonolite specimens sell for around $1, typical collector pieces about $10, and premium examples $75 or more depending on size, quality and locality.
What color is Phonolite?
Phonolite is most often gray, greenish gray, brown, and black. Color varies with impurities and locality, so confirm with hardness and luster rather than color alone.
