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.
If this hole is later filled with other minerals, it is a cast.
An endocast or internal mold is formed when sediments or minerals fill the internal cavity of an organism, such as the inside of a bivalve or snail or the hollow of a skull. If the chemistry is right, the organism (or fragment of organism) can act as a nucleus for the precipitation of minerals such as siderite, resulting in a nodule forming around it.
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.
This process can occur in very small spaces, such as within the cell wall of a plant cell.
Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, hair, petrified wood, oil, coal, and DNA remnants.The empty spaces within an organism (spaces filled with liquid or gas during life) become filled with mineral-rich groundwater.Minerals precipitate from the groundwater, occupying the empty spaces.Often what remains is a carbonaceous film known as a phytoleim, in which case the fossil is known as a compression.Often, however, the phytoleim is lost and all that remains is an impression of the organism in the rock—an impression fossil.