A fossil is the remains, traces, or impression of a living organism that inhabited the Earth during past ages (at least 10,000 years ago). Traces, may be, for example, footprints, burrows, or preserved droppings.
Fossil remains take a number of forms. Petrification describes two ways in which the shape of hard parts of the organism may be preserved. In permineralization, the pore spaces of the hard parts are infilled by certain minerals (for example, silica, pyrite, calcite) that infiltrate from the local groundwater. The resulting fossil is thus a mixture of mineral and organic matter. In many other cases, mineralization (or replacement) occurs, where the hard parts are dissolved away but the form is retained by deposited minerals. Where this has happened very gradually, even microscopic detail may be preserved; but generally only the outward form remains.
Exceptional fossils are those where the organism has been preserved in its entirety: for example, mammoths in the Siberian permafrost, or insects in amber (though dehydrated). Sometimes teeth and shells may be preserved unaltered.
Often the organism is dissolved entirely, so that only a cast or impression remains in the rock. In the process of carbonization, the tissues decompose leaving only a thin carbon film that shows the outline of the organism's form.