Granite: Formation & Identification

01 / Origin story

How does granite form?

Granite is an intrusive igneous rock — it forms when magma cools slowly, deep underground, over very long timescales. That slow cooling is why granite has such distinctly visible crystals; the minerals had time to grow large before the rock fully solidified.

02 / Read the clues

How can you identify granite?

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

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

01Coarse-grained — individual mineral crystals are visible to the naked eye
02Typically light-colored: shades of pink, gray, or white, often speckled with black
03Hard — will scratch glass
04Made mostly of quartz and feldspar, with some mica or hornblende giving it darker flecks

03 / Commonly confused with

Similar at first glance.

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

Granite specimen for comparison — Granite ↗
Coarse-grained — individual mineral crystals are visible to the naked eye

Gneiss specimen for comparison — Gneiss ↗
Bold, coarse banding, alternating light (quartz/feldspar) and dark (mica/hornblende) layers

Gneiss (a metamorphic rock) — both can look similarly speckled and coarse-grained, but gneiss has visible banding/layering from being deformed under pressure, while granite's crystals are randomly arranged with no banding.

04 / Out in the world