The websites About.com and Geology.com provide rock identification charts. The rock chart at About.com includes a brief description of the rock's color, grain size and composition. Geology.com features a picture of each of the rocks included in the chart; more detailed information on each rock is available by clicking on the rock's image.
Rock identification charts classify rocks according to their appearance, composition and formation. There are three main types of rocks, attending to their formation process: igneous, sedimentary and metamorphic.
Igneous rocks are formed from solidification of molten material below Earth's surface, forming crystals. Intrusive igneous rocks crystallize below Earth's surface, while extrusive igneous rocks crystallize on Earth's surface. Igneous rocks include basalt, granite and obsidian.
Sedimentary rocks form from the accumulation of sediments. There are three main types: clastic, chemical and organic. Clastic sedimentary rocks are formed from mechanical weathering debris. Chemical sedimentary rocks are formed when materials that are dissolved in the water precipitate from solution. Organic sedimentary rocks are formed from animal or plant debris. Sedimentary rocks include sandstone, iron ore and coal.
Metamorphic rocks are formed under Earth's surface, where the heat and pressure modify them. Foliated metamorphic rocks have a layered appearance, while non-foliated metamorphic rocks do not. Metamorphic rocks include marble and gneiss.