Wood Atlas - Xylarium
The bog oak (Quercus robur) is not a separate species from the English oak, but instead refers to logs that have spent centuries, even millennia in the anaerobic conditions of bogs and swamp. The tannic acid reacts with the iron salts contained in the water, which not only discolors the wood, but also makes it very hard. The coloration can be very irregular, varying from light grey to dark yellow, dark brown, or from blue-grey to deep black. These partially fossilized oaks can be between 600 and 8,500 years old.