Mile, I hope you don't mind me adding more about this really great wood type.
I was under the impression these trees are not necessarily oak. They are just called that name for some reason.
This is true. Some are oaks, others are acacia or cypress. Even pine is occasionally found. The common name given is "Bog Oak", although the proper name is "Bog Wood".
The term "Bog" rarely has anything to do with peat bogs, which produce a different looking wood. Any mud/silt/clay/wet soil covered log that semi-petrifies as the cellulose converts to carbon, is said to be recovered from a "bog". If it is recovered from under water and not completely covered by the mud/silt, it is called "sinker" wood. The key to the slow and lengthy process is no light - no oxygen - no bacteria/fungus.
Wood from peat bogs has a much higher tannin level, and usually a deep brownish to black look. Wood that is actual oak and was in swamps or other tannin rich waters also gets quite dark. Other woods and locations vary in color and look.
Since it take an expert to tell them apart and identify the species, most are just called "Bog Oak" by wood sellers.
Mile (RedBeardRum) is in one of the richest areas - Croatia, where some very high grade wood is recovered from ancient stream beds. The type of soil/clay in the river beds, plus the constantly changing stream path over millennia has made large areas of buried tree trunks and roots. Most of the material like what is listed for sale here that I have seen tested is in the 4000-7000 year old range. By 8000 years it gets a solid carbon black color, and is called "jet" (thus, the color name "jet black"). Much older and it starts becoming coal .... or disintegrates with age. The material found under water is often newer, and can run between 100 and 3000 years old. It goes from tan to brown in color.
A related wood to this type is ancient kauri, from New Zealand. These huge trees were thousands of years old when they fell into peat bogs and swamps during some cataclysm 50,000 years ago ( perhaps a meteor, earthquake, or a gigantic cyclone). The wood looks as if fresh cut from a modern tree - only better. It gets iridescent when sanded and finished right. Some trunks are so huge they cut 8 foot wide boards from them. The smaller chunks and pieces are often used by wood turners ... and knifemakers.
Extra Tidbit - IIRC, it is pipe carvers who really prize the oldest and blackest "bog oak" .... especially the gnarly pieces from root burls. It is supposed to make some of the best pipes as the wood is already carbonized. This grade wood also polishes wonderfully.