It is a concretion commonly referred as a septarian nodule.
Septarian concretions or septarian nodules, are concretions containing angular cavities or cracks, which are called "septaria". The word comes from the Latin word septum; "partition", and refers to the cracks/separations in this kind of rock. (from Google)
Is that paint or pigment on that stone, it almost looks like a rune symbol. Was it near ancient human settlement?