These magical and extremely intriguing Boulders are known as cannon ball concretions made from mudstone. Some are particularly large.
It is believed that millions of years ago rivers carried sediment from eroding land out to sea. Shells and trees were also carried out to sea and were trapped in the sediment. The sea water contained dissolved chemicals and formed a cement around what was left of the buried shells and plant material. The cement in this case is calcium carbonate. The organic material seems to create a node for this process to occur.
This is why some are split and some even have trees growing from the centre.