What are most fossils made up of?

Most fossils are made up of minerals, such as calcite, silica, or pyrite. These minerals replace the original organic material of the organism over time. This can happen when the organism is buried in sediment and exposed to groundwater that is rich in dissolved minerals. The minerals slowly infiltrate the organism's tissues and eventually replace them. In other cases, the organism may be exposed to hot volcanic ash or lava, which can quickly vaporize the organic material and leave behind a mineral cast. This can happen either at the time of a volcanic eruption or as part of other rock-forming processes. Additionally, some fossils are preserved in amber (fossilized plant resin), while others may be preserved in tar pits or in peat bogs.