Identification guide · Igneous Rocks
How to Identify Gabbro
Gabbro shows a coarse, phaneritic texture with visible interlocking crystals of dark green to black pyroxene set against light to medium gray plagioclase feldspar, giving the rock an overall dark, speckled appearance. Its luster varies mineral by mineral, vitreous on feldspar cleavage faces and duller on pyroxene, and its streak is generally white or gray since testing the whole rock produces a mix of its component minerals. Hardness runs around 6, and gabbro is commonly confused with basalt, which shares its composition but is fine-grained, or with diorite, which is lighter overall because it contains more feldspar and less pyroxene.
Gabbro identification checklist
| Color | Dark gray, Black, Greenish black, Speckled black and white | Look at a clean, dry surface in daylight. Color zoning, banding or spots are as diagnostic as the hue. |
|---|---|---|
| Luster | Dull to vitreous (in crystals) | 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 | 5–6.5 | It is about as hard as window glass: a knife barely marks it, while a steel file (Mohs 6.5) will. |
| Fracture | Uneven, granular | |
| Specific gravity | 2.9–3.1 | Ordinary rock-like heft (specific gravity about 3), similar to quartz. |
Gabbro look-alikes and how to tell them apart
Both sit near Mohs 5–6.5, so a scratch test alone will not separate them. Lean on luster, streak and heft instead. Gabbro vs Andesite →
Both sit near Mohs 5–6.5, so a scratch test alone will not separate them. Lean on luster, streak and heft instead. Gabbro vs Diorite →
Both sit near Mohs 5–6.5, so a scratch test alone will not separate them. Lean on luster, streak and heft instead. Gabbro vs Rhyolite →
Both sit near Mohs 5–6.5, so a scratch test alone will not separate them. Lean on luster, streak and heft instead.
Gabbro identification FAQ
How can you tell if a rock is Gabbro?
Check the color (dark gray, black, greenish black, and speckled black and white), the dull to vitreous (in crystals) luster, a Mohs hardness of 5–6.5. Photographing it with the StoneSnap app gives a match in seconds.
What is the hardness of Gabbro?
Gabbro has a Mohs hardness of 5–6.5. 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 Gabbro and Andesite?
Gabbro (Mohs 5–6.5, dull to vitreous (in crystals) luster) and Andesite (Mohs 5–6, dull groundmass with vitreous phenocrysts luster) look alike, but hardness, streak and heft usually separate them. See the full Gabbro vs Andesite comparison for a side-by-side table.
Is Gabbro worth anything?
Entry-level Gabbro 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 Gabbro?
Gabbro is most often dark gray, black, greenish black, and speckled black and white. Color varies with impurities and locality, so confirm with hardness and luster rather than color alone.
