Identification guide · Igneous Rocks
How to Identify Tonalite
Tonalite is usually white, gray, and black 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 white to light gray. It is most often confused with Monzonite, Granite, and Aplite; the tests below separate them.
Tonalite identification checklist
| Color | White, Gray, Black, Salt and pepper, Greenish 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 | White to light gray | Drag a corner across unglazed porcelain (the back of a bathroom tile) and check the powder color. |
| Cleavage | Variable by feldspar grains | |
| Fracture | Uneven to granular | |
| Specific gravity | 2.75 | Ordinary rock-like heft (specific gravity about 2.75), similar to quartz. |
Tonalite 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. Tonalite vs Monzonite →
Both sit near Mohs 6, so a scratch test alone will not separate them. Lean on luster, streak and heft instead. Tonalite vs Granite →
Both sit near Mohs 6, so a scratch test alone will not separate them. Lean on luster, streak and heft instead. Tonalite vs Aplite →
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 coarse gray-white-black texture with quartz and abundant plagioclase.
- Tonalite has less pink alkali feldspar than many granites.
- It can sit visually between diorite and granodiorite.
- A close photo of grain mix helps avoid overcalling granite.
Tonalite identification FAQ
How can you tell if a rock is Tonalite?
Check the color (white, gray, black, and salt and pepper), the dull luster, a Mohs hardness of 6 and a white to light gray streak. Photographing it with the StoneSnap app gives a match in seconds.
What is the hardness of Tonalite?
Tonalite 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 Tonalite and Monzonite?
Tonalite (Mohs 6, dull luster) and Monzonite (Mohs 6, dull luster) look alike, but hardness, streak and heft usually separate them. See the full Tonalite vs Monzonite comparison for a side-by-side table.
Is Tonalite worth anything?
Entry-level Tonalite 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 Tonalite?
Tonalite is most often white, gray, black, and salt and pepper. Color varies with impurities and locality, so confirm with hardness and luster rather than color alone.
