Fossils are almost always found in sedimentary rock strata. Sedimentary rocks are the most abundant type on the surface of the Earth, and they are the only type that forms under conditions that are likely to preserve organic artifacts.
Non-sedimentary rocks are either unable to collect fossils or likely to destroy them during their formation. Igneous rock pours out from the molten region of the Earth's crust and cools at or near the surface. Surface igneous rocks, such as obsidian and pumice, erupt from volcanoes and cool before they can take up organic remains. Any bone, tooth or plant matter they come into contact with while they are still malleable is burned completely, leaving no trace to fossilize. Substrate igneous formations, such as plutonic granite, rise in a hot bolus to a point just beneath the surface and cool in place, which prevents biological material from entering the rock at any point.
The third rock type, metamorphic, is occasionally found with fossils embedded in it, but this is unusual. Metamorphic rock usually begins as a sediment, such as limestone, but undergoes a process of reformation under conditions of extreme heat and pressure. Metamorphic conditions often destroy fossils, and the resulting rock, such as marble, rarely has any remaining trace of biological entities.