Identification guide · Igneous Rocks
How to Identify Basalt
Basalt is usually black, dark gray, and dark green 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; test individual minerals. It is most often confused with Monzonite, Tonalite, and Norite; the tests below separate them.
Basalt identification checklist
| Color | Black, Dark Gray, Dark Green | 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; test individual minerals | Drag a corner across unglazed porcelain (the back of a bathroom tile) and check the powder color. |
| Cleavage | Variable by mineral grains | |
| Fracture | Conchoidal | Broken surfaces show smooth, curved, shell-like scoops. |
| Specific gravity | 3 | Ordinary rock-like heft (specific gravity about 3), similar to quartz. |
Basalt 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. Basalt vs Monzonite →
Both sit near Mohs 6, so a scratch test alone will not separate them. Lean on luster, streak and heft instead. Basalt vs Tonalite →
Both sit near Mohs 6, so a scratch test alone will not separate them. Lean on luster, streak and heft instead. Basalt vs Norite →
Both sit near Mohs 6, so a scratch test alone will not separate them. Lean on luster, streak and heft instead. Basalt vs Phonolite →
Field tips
- Check whether the specimen's color and texture match typical Basalt examples.
- Use hardness, streak, luster, grain size, and locality together; no single trait is enough for every specimen.
Basalt identification FAQ
How can you tell if a rock is Basalt?
Check the color (black, dark gray, and dark green), the dull luster, a Mohs hardness of 6 and a variable; test individual minerals streak. Photographing it with the StoneSnap app gives a match in seconds.
What is the hardness of Basalt?
Basalt 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 Basalt and Monzonite?
Basalt (Mohs 6, dull luster) and Monzonite (Mohs 6, dull luster) look alike, but hardness, streak and heft usually separate them. See the full Basalt vs Monzonite comparison for a side-by-side table.
Is Basalt worth anything?
Entry-level Basalt specimens sell for around $1, typical collector pieces about $5, and premium examples $30 or more depending on size, quality and locality.
What color is Basalt?
Basalt is most often black, dark gray, and dark green. Color varies with impurities and locality, so confirm with hardness and luster rather than color alone.
