Sedimentary Rocks
Sandstone	Appearance: looks just like compressed sand. Grainy to the touch. Sandstone may have layers.
Limestone	Appearance: yellowy-grey-greenish. It will bubble (slightly) when contacted by acid. This is often made from shells and frequently has fossils.
Shale	Appearance: ordinary looking grey rock. Compacted from mud, some shales are rich with oil. Fossils frequent.
Geode	Hollow rocks, often with crystallized minerals inside. The outside hardened around water, and the minerals formed inside. Geodes can carry a variety of minerals.
Conglomerate	Appearance: looks like a bunch of rocks cemented together.
Organic Rocks (rocks from living things)
Coal	Appearance: glossy black, but not quite like glass. Coal breaks in chunks. Made from trapped plant matter.
Coral	Appearance: chalky, often with holes. Coral is calcium carbonate and is really the skeletons of sea animals.
Petrified wood	Appearance: in the shape of wood, but clearly is stone. Made when minerals replace the living tissues of trees. Usually happens under water. Also called permineralization.