These are not rocks, but porous fungi that grow on birch trees, drawing nutrients out and storing them. This activity is part of the trophic chain: as old forms of life begin to die, fungi will piggyback on them to extract riches with which to replenish the soil. Chaga mushrooms are wondrous -- not for eating, but from which to make a tea that you can drink daily or use as a stock in soups. (Boil one lump in a big pot of water and simmer for 1-2 hours. Refrigerate, dilute if desired to drink or use. Put the "rock" in the freezer and re-use several times.)
Chagas do so much for us -- impossible to list it all, so read more about them here!