Identification guide · Igneous Rocks
How to Identify Ignimbrite
Ignimbrite is usually tan, pink, and gray 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 variable; often light gray or tan. It is most often confused with Pumice, Basaltic Andesite, and Trachyte; the tests below separate them.
Ignimbrite identification checklist
| Color | Tan, Pink, Gray, White, Red brown, Black | 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 | Variable; often light gray or tan | Drag a corner across unglazed porcelain (the back of a bathroom tile) and check the powder color. |
| Cleavage | None | |
| Fracture | Uneven to fragmental | |
| Specific gravity | 2.25 | Noticeably light in the hand for its size (specific gravity about 2.25). |
Ignimbrite 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. Ignimbrite vs Pumice →
Both sit near Mohs 6, so a scratch test alone will not separate them. Lean on luster, streak and heft instead. Ignimbrite vs Basaltic Andesite →
Both sit near Mohs 6, so a scratch test alone will not separate them. Lean on luster, streak and heft instead. Ignimbrite 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 ash-rich volcanic texture with pumice or rock fragments.
- Flattened dark or light lenses can indicate welded ignimbrite.
- Ignimbrite can resemble ordinary tuff, so welding texture and fragment shapes matter.
- A close-up of the matrix and fragments improves identification.
Ignimbrite identification FAQ
How can you tell if a rock is Ignimbrite?
Check the color (tan, pink, gray, and white), the dull luster, a Mohs hardness of 6 and a variable; often light gray or tan streak. Photographing it with the StoneSnap app gives a match in seconds.
What is the hardness of Ignimbrite?
Ignimbrite 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 Ignimbrite and Pumice?
Ignimbrite (Mohs 6, dull luster) and Pumice (Mohs 5–6, dull to glassy luster) look alike, but hardness, streak and heft usually separate them. See the full Ignimbrite vs Pumice comparison for a side-by-side table.
Is Ignimbrite worth anything?
Entry-level Ignimbrite 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 Ignimbrite?
Ignimbrite is most often tan, pink, gray, and white. Color varies with impurities and locality, so confirm with hardness and luster rather than color alone.
