The grain size of crystalline igneous rocks can be
separated into three broad categories.
Coarse-grained igneous rocks have individual
grains (crystals) that are visible to the naked eye.
Grains can range from millimeters to centimeters in
diameter. The grains form an interlocking network.
Fine-grained igneous rocks have small grains,
less than a millimeter across, that are invisible to
the naked eye. Hand samples of fine-grained rocks often
appear homogeneousthey look the same throughout
the rock. Hand lenses, magnifying glasses, or microscopes
can reveal the small crystals in these rocks.
Igneous rocks with a porphyritic texture show
two distinct grain sizescoarse-grained crystals,
visible to the naked eye, surrounded by fine-grained