Plant fossils are usually created through carbonization. During this process, all the organic residue of the plant decomposes and all that remains is a film of carbon. Over eons, this carbon carves a detailed impression into the rock and a fossil is created and preserved for all time.
Petrification is another way that fossils are formed. The best known form of petrified fossils is the Petrified Forest in California. During this process, minerals inside the plant or tree cells become crystals. Silica-rich fluids enter the tree's cells and slowly crystallize, converting the tree into a stone-like structure. Shells can also be petrified.
Some ancient ice-age mammals like the hairy mammoth and woolly rhinoceros have been preserved in remarkable condition when they flash-froze for some unknown reason. Some of these animals are so well preserved that paleontologists have been able to clearly distinguish the color of their skin and hair and even the contents of their stomachs. This fossilization process is limited to areas of deep ice.
Tree resin that has become fossilized is known as amber and prehistoric insects have often been trapped and fossilized in amber. When resin first starts flowing from the tree, it is sticky and viscous and insects like flies and spiders or even small animals like frogs get stuck in this gel-like glob. Over time, as the amber becomes fossilized, the trapped organisms become fossilized as well.