Ancient Oak trees (quercus spp.) go through a process making them very different from our modern oak lumber.  The wood goes through thousands of years of maturation underground in bogs, swamps, river bottoms, etc. and changes color over time, turning grey or black and the density increases.   The color depends on the mineral content the wood is exposed to over many years without oxygen.   Some grey pieces may be older then the black ones, as some mineral salts don't allow a change to black but the wood nevertheless gets an older carbon date.   In general, the darker the piece the older it is as a best guess.   Unless otherwise identified, the wood we are selling is from Ukraine, and is usually from salt bogs so it will also contain some ancient salt deposits.  This is the type we call Saltwater Bog Oak.    The salt is unique and some artisans choose to leave it in the grain, others remove it.  If you want to remove it, I have had good luck first knocking the salt crystals loose with a pin head and then running tap water over the piece and brushing with a toothbrush if necessary.   You can then dry it off quickly and place for an hour or so in front of a fan or other moving air to get rid of the surface moisture.  These do come with a little information certificate, if you want it just request it in the shopping cart.