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Instructions
Confirm that you are looking at an igneous rock. Sedimentary rocks have layers. Metamorphic rocks have well-organized crystals or crystals of equal size. Igneous rocks can have visible crystals, but they are not orderly like metamorphic rock.
Observe the texture of your specimen. There a few possible textures for igneous rock. Phaneritic rocks have clearly defined crystals, while aphanitic rocks have a smoother texture because the crystals are less visible. Sometimes even aphanitic rocks have large, visible crystals within them, but the rock surrounding the visible crystals appears smooth. There are two smaller texture categories: glassy and vesicular, which contain holes caused by bubbles that formed in the hardening rock.
Identify the minerals present in your specimen. This takes practice, but once you can readily identify minerals you can start remembering the mineral combinations that make up igneous rock specimens. Some important minerals to know are amphibole (black, elongated), biotite (black), muscovite (silver or golden), olivine (light green), plagioclase (white), potassium feldspar (pink), pyroxene (dark green) and quartz (clear, glassy).
Memorize the mineral combinations that make up different kinds of igneous rock. If you can't be sure in the field bring a specimen home or jot down some notes and consult a field guide.
Make note of colors, location and other rocks near where you found the specimen.