Igneous Rocks - Rocks formed from cooled  molten material 

Classified by:

Origin

Intrusive - Cools under the surface of Earth

Cools slowly thus larger crystals 

Extrusive -Cools on the surface of Earth (obsidian, pumice, basalt)

Cools quickly thus small or no crystals (granite)

Texture (grain size)

no grain - cools so quickly no crystals form (obsidian)

fine grain - cools fast so crystals are small (rhyolite, basalt, pumice)

coarse grain - cools slowly so crystals are large (granite)

porphyritic - cools slowly, then quickly, so crystals are large and small

Mineral Composition

high silica content yields light-colored rock

low silica content yields dark-colored rock

Some uses for some igneous rocks

Basalt - road construction

Pumice - polishes

Granite - building materials