Bog-wood is created from the trunks of trees that have lain in bogs, and bog-like conditions such as lakes, river bottoms and swamps, for centuries and even millennia. Deprived of oxygen, the wood undergoes the process of fossilization.
Water flow and depth play a special role in the creation of bog-wood. Currents bind the minerals and iron in the water with tannins in the wood, naturally staining the wood in the process. This centuries-long process, often termed "maturation," turns the wood from golden-brown to completely black, while increasing its hardness to such a level that it can only be carved with the use of specialty cutting tools.
While the time necessary for the oak to transform into bog-wood varies, the "maturation" commonly lasts thousands of years. Due to the ecological reasons mentioned above, no two trunks can be found of the same color.
 
Source : wikipedia