A living animal dies and becomes trapped in a layer of sediment: this is what a fossil is. A rock--a hard-form combination of earth's minerals--sometimes captures a piece of animal bone, an imprint of a leaf, or a whole insect's shell inside the rock form. With layers of sediment around the rock and on top of bone or plant imprints, it is difficult to tell whether a rock is sand, mud and minerals in an intriguing shape or if the rock actually holds a prehistoric (or just historic) fossil matter. If you have a rock to identify, a few steps can help you understand that rock's story.
Decide whether the rock is only a rock or rock fossil. There are three types of rock-- sedimentary, igneous, and metamorphic. Check if the rock has stripes: sedimentary rocks--made of compacted particles of other rocks or mud--appear striated and often multicoloured. Igneous rocks are made of cooled volcanic lava and magma (molten rock) and look hard and polished like granite. See if your rock has crystals: these gems gradually form on cooling magma. Metamorphic rocks form when rocks deep in the ground squeeze during mountain formation or compress in great heat. If your rock is smooth and shiny, like marble, it's a metamorphic rock. All of these rocks may or may not have fossils inside them.
Identify the rock as a fossil. Fossils usually show imprints of hard-parts from dead animals or plant matter like shells, bones, teeth, or plants. Examine the rock closely to look for the ribs of a shell or the detailed imprint of a set of leaves. If you notice that the imprint is set deeply in the rock this may make it a fossil specimen valuable to geologists and historians. Rocks that have actual bits of the animal still present in the rocklike an ant trapped in chunk of amber or a frond leaf paused in a piece of quartz--are also valuable rock-fossils.
Determine the rock material. Study the rock to find out its composition. For instance, limestone (a sedimentary rock) is made up entirely of skeletal fragments of marine organisms. If you have a limestone rock, check the sides of the rock for whole chunks of shell, detailed shell imprints, or coral markings clearly visible all around the rocks edges. Rocks made of hardened lava (Igneous or granite rock) do not normally hold fossils. If you have a smoother, shiny rock like granite that looks like it has a fossil inside it, it is most likely a sedimentary rock that got caught in lava and then developed a hard shiny outer igneous-coating. Geologists rarely find fossils in metamorphic rocks--like marble that is compressed sedimentary rock. However, search a smooth metamorphic rock for markings of bone and teeth matter. According to Science Magazine (http://www.sciencemag.org/cgi/content/abstract/sci;262/5134/732), in 1993 geologists discovered metamorphic rocks off the Coast range of Venezuela with fossils from the Jurassic era.
Determine what type of fossil is inside the rock. Geologists classify fossils in 5 distinct categories: plant (leaf or stem imprint), shell (either whole or a few rib imprints), tooth (the entire rock or an imprint of it), scales (imprints of fish or lizard scales), or unidentifiable--a imprint of piece of matter that an onlooker cannot readily identify. Fossil experts sometimes find skin, feathers, and hair trapped in a rock formation.
Occasionally, in environments where a sudden onslaught of sediment has covered an area very quickly (like in a volcanic eruption or tar pit) soft animal parts--like skin, feathers, fur, tissues, or flowers--can stay preserved inside a rock. Fossils tell us about what animals and plants looked like millions of years ago; they also help scientists study evolution.