Gabbro: Formation & Identification

Q: How can you identify gabbro?

Coarse-grained, like granite or diorite, but dark Dark gray to black, sometimes with a slight greenish tint Heavy — dense, iron/magnesium-rich minerals

01 / Origin story

How does gabbro form?

Gabbro is the intrusive (slow-cooling, underground) counterpart to basalt — same dark, iron-and-magnesium-rich composition, but cooled slowly enough deep in the crust to grow large crystals instead of basalt's fine grain.

02 / Read the clues

How can you identify gabbro?

Use several clues together.
One photograph shows one specimen, not every variation.

Enlarged crop of the Gabbro specimen — Texture study · enlarged crop of this specimen

01Coarse-grained, like granite or diorite, but dark
02Dark gray to black, sometimes with a slight greenish tint
03Heavy — dense, iron/magnesium-rich minerals

03 / Commonly confused with

Similar at first glance.

Compare the materials side by side.
Photographs are not shown to the same scale.

Gabbro specimen for comparison — Gabbro ↗
Coarse-grained, like granite or diorite, but dark

Basalt specimen for comparison — Basalt ↗
Fine-grained — crystals are too small to see individually, often looks almost glassy or dull

Basalt — chemically the same rock, different cooling history. If you can see individual crystals with the naked eye, it's gabbro; if it looks smooth/fine and almost featureless, it's basalt.

04 / Out in the world