How Does a Bone Turn into Stone?

The Answer

Water seeping through the ground carries dissolved minerals into the buried bone. Over a very long time the minerals fill every gap and slowly replace the bone itself, leaving a stone copy.

First the animal has to be buried quickly, by mud, sand or volcanic ash, before scavengers and rot can destroy it. Soft parts usually rot away anyway, which is why most fossils are bones, teeth and shells.

Groundwater is never pure water. It carries minerals dissolved from the rocks it has passed through, such as silica (the stuff of sand) and calcite. As it seeps through the buried bone, the minerals crystallise in the tiny holes and channels and fill them with stone. Over thousands to millions of years the bone's own material also dissolves away bit by bit, and minerals take its place. The shape survives, but the bone has become rock. Petrified wood forms in the same way.

A kettle in a hard-water area shows a tiny version of the process: the water leaves behind a crust of limescale, made of minerals that were dissolved in it. Fossils are rare, though. Almost every animal that has ever lived rotted away or was eaten without leaving a trace.

The principle: Fossilisation by mineral replacement. Groundwater flowing through buried remains deposits minerals in their spaces and slowly swaps the original material for stone, keeping its shape.