Most fossils undergo some alteration, though in many cases it may only be possible to see these changes under a microscope. The changes can be both chemical and physical. Carbonisation means the organism is 'altered' by being reduced to carbon. This occurs in oxygen free environments such as deep, stagnant water. One of the best examples of carbon is coal.
Coal consists of plants which have been reduced to almost pure carbon.
Skeletons of fossil fish can be found with their bones completely covered in carbonaceous matter. This happens when the fossil is buried in wet sediment, such as an ocean bed and the sea water seeps through the surrounding rock and alters the fossil. The microscopic structure of the fossil's tissues is usually destroyed by this type of preservation. Silica solutions can also seep through the sediments and replace the hard parts of fossils, even when the fossil is made of the same substance as the surrounding sediment. Silicified fossils in limestone are easily extracted by dissolving the limestone in acid.