The fossils are testimonies from the past. They evidence the existence of organisms in other geological eras and they are present at different levels in the earth layers.
The fossilization process, which takes place along millions of years, consists basicaly in the replacement of the remains of an organism components by minerals.
This way, the fossils have the same shape as the original organism or bone but they are no longer made of the same material. They are heavier and their color and texture vary widely. Most fossils are very similar to rocks, but others may be more exotic, such as opals or amber.
Sometimes, fossils are not remains but evidence of the activity of live beings, such as burrows, footprints, track ways or perforations.
Some fossil findings have taken place by chance. But, as most fossils are generally buried, it may be tough to decide the exact point for an excavation.