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(Below) The stages in making a cast or a mould of a fossil.
away and replaced by silica. They are now real stone, but the replacement is so perfect that the features of the original trees can still be easily made out.
Silica is just one of many materials that can replace the hard parts of a fossil. Pyrite is a yellow mineral that is commonly found in muds where the oxygen is poor. It can replace the shell of fossil sea-dwelling creatures with casts that are exact replicas of the original.
Even if there are no solutions replacing
the dissolved hard parts, the rock may simply
preserve the shape of the hard parts as moulds. Insects preserved in amber are usually moulds, although the insects in them are preserved in such minute detail that they seem to be the
original organic materials.
A mould of a thin object, such as a
leaf, is known as an imprint.
 Stage 1. The dead animal is buried by sediment.
 Stage 2. The soft parts, and possibly the skeleton as well, are dissolved away, leaving a mould.
 Stage 3. Solutions passing through the sediment deposit minerals in the mould
to produce a cast of the skeleton (and sometimes also the soft parts) of the original creature. Both mould and cast keep a record of the fossil and may be found in rocks.
Mould
Cast
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