Identification guide · Igneous Rocks
How to Identify Tuff
Tuff is typically light gray, tan, pink, or white, with a rough, porous, and sometimes crumbly texture that reveals visible fragments of pumice, crystals, and rock debris embedded in a fine ash matrix. Hardness varies widely from soft and easily scratched to quite hard in densely welded varieties, so testing depends heavily on the specific sample. It shows no true cleavage, since it is a fragmental rock rather than a single mineral, and it fractures irregularly around its embedded fragments. Tuff is sometimes mistaken for sandstone, but its angular volcanic glass shards and pumice fragments, visible under a hand lens, confirm a volcanic rather than sedimentary origin.
Tuff identification checklist
| Color | Light gray, Tan, Pink, Greenish, White | Look at a clean, dry surface in daylight. Color zoning, banding or spots are as diagnostic as the hue. |
|---|---|---|
| Luster | Dull to earthy; glassy shards may show vitreous luster | Fresh surfaces shine like glass; a dull surface usually means weathering, so chip or wet a corner. |
| Transparency | Opaque | Hold a thin edge up to a bright light and note whether light passes through. |
| Mohs hardness | 2–5 | A steel knife blade (Mohs 5.5) scratches it; a copper coin does not. |
| Fracture | Uneven to conchoidal (in glassy fragments) | Broken surfaces show smooth, curved, shell-like scoops. |
| Specific gravity | 1.5–2.7 | Noticeably light in the hand for its size (specific gravity about 2.1). |
Extra tests for Tuff
Glass check. Tuff 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.
Tuff look-alikes and how to tell them apart
Feldspar is harder (Mohs 6–6.5) than Tuff (Mohs 2–5), so Feldspar will scratch Tuff and not the other way round. A steel knife blade (Mohs 5.5) scratches it; a copper coin does not. Tuff vs Feldspar →
Quartz is harder (Mohs 7) than Tuff (Mohs 2–5), so Quartz will scratch Tuff and not the other way round. A steel knife blade (Mohs 5.5) scratches it; a copper coin does not. Tuff vs Quartz →
Andesite is harder (Mohs 5–6) than Tuff (Mohs 2–5), so Andesite will scratch Tuff and not the other way round. A steel knife blade (Mohs 5.5) scratches it; a copper coin does not. Tuff vs Andesite →
Both sit near Mohs 2–5, so a scratch test alone will not separate them. Lean on luster, streak and heft instead.
Tuff identification FAQ
How can you tell if a rock is Tuff?
Check the color (light gray, tan, pink, and greenish), the dull to earthy; glassy shards may show vitreous luster luster, a Mohs hardness of 2–5. Photographing it with the StoneSnap app gives a match in seconds.
What is the hardness of Tuff?
Tuff has a Mohs hardness of 2–5. A steel knife blade (Mohs 5.5) scratches it; a copper coin does not.
What is the difference between Tuff and Feldspar?
Tuff (Mohs 2–5, dull to earthy; glassy shards may show vitreous luster luster) and Feldspar (Mohs 6–6.5, vitreous to pearly luster) look alike, but hardness, streak and heft usually separate them. See the full Tuff vs Feldspar comparison for a side-by-side table.
Is Tuff worth anything?
Entry-level Tuff 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 Tuff?
Tuff is most often light gray, tan, pink, and greenish. Color varies with impurities and locality, so confirm with hardness and luster rather than color alone.
