Igneous rocks rarely contain fossils because the formation of a fossil requires sedimentation. A fossil results when the remains of a plant or animal are covered by sediment that hardens, forming the mold that eventually fills with minerals. Igneous rocks form under heat and pressure that destroy organic remnants.
For a fossil to form, the remains of a living thing must be buried quickly enough to prevent scavengers from destroying the carcass, and the layers of sediment must harden quickly enough to form a mold around the remains before they decay. When this set of circumstances is achieved, the result is a void in the sedimentary rock that approximates the shape and arrangement of the original remains. Water flows through the rock and deposits minerals into this void, eventually creating a stone representation of the original remains called a fossil.
Igneous rocks form when lava or magma cools, crystallizing into various types of stone. If lava were to flow over the remains of a plant or animal, it would burn away the organic tissue well before it cooled enough to solidify around the remains and create a fossil. Certain types of igneous rocks form from cooler pyroclastic material such as mud and ash, but even these materials are much less likely to create fossils than layered sediment.