What is a petrified rock?

While there's no such thing as petrified rock, petrified wood is fossilized wood. According to YourGemologist, there is no wood actually left in petrified wood, only rock that takes the same form and shape of the wood.

Petrification typically takes millions of years. Normally, trees decompose, but when a tree lands in a swamp or is otherwise covered up, decomposition is halted. The cells within the tree are hollowed out, and mineral-rich water seeps in. As the water evaporates, the minerals are left behind, creating a copy of what was there. It is possible to see growth rings in well-preserved samples.