Identification guide · Igneous Rocks
How to Identify Basaltic Andesite
Basaltic Andesite is usually dark gray, black, 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 gray to dark gray. It is most often confused with Ignimbrite, Norite, and Phonolite; the tests below separate them.
Basaltic Andesite identification checklist
| Color | Dark gray, Black, Brown, Red brown, Gray | 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 | Gray to dark gray | 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 | 2.85 | Ordinary rock-like heft (specific gravity about 2.85), similar to quartz. |
Basaltic Andesite 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. Basaltic Andesite vs Ignimbrite →
Both sit near Mohs 6, so a scratch test alone will not separate them. Lean on luster, streak and heft instead. Basaltic Andesite vs Norite →
Both sit near Mohs 6, so a scratch test alone will not separate them. Lean on luster, streak and heft instead. Basaltic Andesite vs Phonolite →
Both sit near Mohs 6, so a scratch test alone will not separate them. Lean on luster, streak and heft instead. Basaltic Andesite vs Tephrite →
Field tips
- Look for dark gray to black fine-grained volcanic texture.
- Basaltic andesite may show tiny feldspar or pyroxene crystals.
- Vesicles can occur but are not required.
- It is visually between basalt and andesite, so locality and context matter.
Basaltic Andesite identification FAQ
How can you tell if a rock is Basaltic Andesite?
Check the color (dark gray, black, brown, and red brown), the dull luster, a Mohs hardness of 6 and a gray to dark gray streak. Photographing it with the StoneSnap app gives a match in seconds.
What is the hardness of Basaltic Andesite?
Basaltic Andesite 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 Basaltic Andesite and Ignimbrite?
Basaltic Andesite (Mohs 6, dull luster) and Ignimbrite (Mohs 6, dull luster) look alike, but hardness, streak and heft usually separate them. See the full Basaltic Andesite vs Ignimbrite comparison for a side-by-side table.
Is Basaltic Andesite worth anything?
Entry-level Basaltic Andesite 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 Basaltic Andesite?
Basaltic Andesite is most often dark gray, black, brown, and red brown. Color varies with impurities and locality, so confirm with hardness and luster rather than color alone.
