Petrification – the impregnation with minerals such as silica or calcite from the surrounding sediments which increases the weight and hardness of the fossil.

Replacement – where the original material of the organism may have dissolved away and the space left filled with minerals from the surrounding rocks. These can be very beautiful especially if the replacing material is silica or pyrite.

Carbonization – the process where the tissue of the organism may be converted into a film of carbon and can be seen in some plants and planktonic animals.