Fossils are rarely found in igneous rocks due to the formation process of these rocks. Igneous rocks are created from the cooling and solidification of molten magma or lava. This process occurs at extremely high temperatures, often exceeding 1000 degrees Celsius.
When an organism dies, the conditions required for fossilization usually involve burial in sediments, where minerals can gradually replace the organic material over time in a process known as permineralization. Sedimentary rocks, which are formed from the accumulation of sediments, provide the ideal environment for this slow and delicate process to occur.
In contrast, igneous rocks form too quickly and at temperatures too high for any organic remains to survive. If any organic material were present when the magma is formed, it would be vaporized before it could form into a fossil. This is why, despite their importance in the geological record, fossils are almost exclusively found in sedimentary rocks, while igneous rocks typically contain no biological remnants.