Fossils are all organic materials such as human bones, animal bones, or the plant due to fossilization process. Fossilization is the process of replacement of organic material (bone, teeth, and wood) to be inorganic in the form of mineral (such as silica and iron oxide) from the sedimented-soil. In this process, the organic elements from bones or woods will be decomposed in time, and replaced by those minerals, becoming harder, heavier, and experiencing color changes suitable with mineral. Whenever all the organic material have completely been changed into minerals, so a bone or wood is regarded as fossil.
In fossilization process, the original form (structure) is persisting, but the composition of bone (texture) turns to be mineral and fossilized. (Harry Widianto dan Iwan Setiawan Bimas)