Wood which, after it dies, is covered up by something such as volcanic ash, volcanic mudflows, sediments in lakes or materials washed in by floods. When it's underground, the wood does not decay because oxygen and microorganisms can't reach it. Minerals dissolved in groundwater get inside the cells of the wood and form chalcedony, other quartzes, or opal, with colors caused by other minerals that are mixed with the silica forming most types of petrified wood. It's most commonly brown but can be many colors. The grain of the wood can still be seen in the rock, and it can be polished up for use in jewelry or other decorative items.