Pick up a fossil skull and you’ll notice it’s surprisingly heavy—as heavy as a rock. That’s because it is a rock, more or less, though it still contains some of the original bone material.
A fossil is a preserved record of a formerly living creature. For a dead creature to fossilize, two things must happen.
First, it has to be buried somehow, before it decomposes completely. Many of the best fossils are those that died in or near water and were soon after covered by mud or sand. The soft parts rotted, but the hard parts—the skeleton—remained.
The next step is mineralization. Over time, water seeps through the skeleton and replaces the bone—and the tiny open spaces within the bone—with minerals like calcite or silica. The skeleton slowly turns to stone.