Fossils can be formed by amber that is tree sap it forms insect fossils, and leaf fossils.Fossils form when an animal dies in the desert or gets frozen in ice or gets trapped in a tar pit. Other types of fossils form when an animal dies and is immediately covered by earth, mud, or sand. The covering slows decay down to a crawl and eventually turns to solid rock. Sometimes water will bubble up through the earth filling a mold with minerals in the shell that makes it a mold fossil.