Fossils are the remains of plants and animals that have died long ago and have been preserved in rock. A fossil can be the mineral replacement of the animal (or plant) itself or the impression of the plant or animal. A fossil may also be the tracks or marks made by and animal while it was alive (known as trace fossils).

In order for a living organism to become fossilized, it's demise must meet certain conditions.

• It must be buried quickly in sand, mud, or ash
• It must be protected from the elements (sun, air, water)
• It must be safe from predators and scavengers (so as not to become a meal)
• It must be free from the disruption of earthquakes and volcanoes

After millions of years, the remains (bones, shells, etc.) undergo a chemical change. The remains are slowly replaced with minerals that form rocks. On rare occasions, the remains are replaced with minerals that form gem stones (such as opal) !