More likely it means that the "boulder" was cracked or split at one time, and ground water with dissolved minerals seeped in and crystalized/solidified over time. IME, that thin vein goes all the way through at the same thickness.
It's some kind of mineralization that formed in a cracked rock formation long before it became a boulder. My wild guess it be manganese in the crack. No one can ever say for sure with only a picture.