It is very interesting! First thing is to determine the type of rock. Igneous, Metamorphic or Sedimentary. Each can define its origins and perhaps the cause of such a interesting outcome. My first thought is that it is of material than formed around another object that erroded over time because the other substance did not come from the same origins. Most likely the remains could have been formed from any one of the Three rock type. I would be curious to know if there were other rocks of whatever size of the same look and this one or weither it was transported by nature to this site. Thanks for showing it. Og