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Some fossils consist only of skeletal remains or teeth; other fossils contain traces of skin, feathers or even soft tissues. In some cases the original remains of the organism completely dissolve or are otherwise destroyed.
The remaining organism-shaped hole in the rock is called an external mold.
The degree to which the remains are decayed when covered determines the later details of the fossil.
In many cases, however, compressions and impressions occur together.
For instance, when the rock is broken open, the phytoleim will often be attached to one part (compression), whereas the counterpart will just be an impression.
A shell is said to be recrystallized when the original skeletal compounds are still present but in a different crystal form, as from aragonite to calcite.
Compression fossils, such as those of fossil ferns, are the result of chemical reduction of the complex organic molecules composing the organism's tissues.