Coal is made from the squashed remains of swampy forests that once covered large areas 300 million years ago. Conglomerate is a solid mass of round pebbles cemented together. It was probably made from beach pebbles. Sandstone is coarse grains of quartz sand, cemented together by silica or calcite. Chalk is a shelly limestone, made from the skeletons, called coccoliths, of microscopic sea creatures that lived in warm seas 65-144 million years ago.
Igneous rocks have a hard, mottled, crystalline look.
Sedimentary rocks are much duller and softer, and you can often rub off the grains. Metamorphic rocks are usually tough and shiny, with grains so small you can hardly see them. Igneous rocks Magma forms various kinds of igneous rock according to how quickly it cools and how acidic and sticky it is. Large intrusions cool slowly underground to form large-grained rocks such as gabbro from basic magma and granite from acidic magma. Polished marble Like many igneous and metamorphic rocks, marble can be ground and polished smooth.