Really Cool Rocks
There is a natural phenomenon called “concretion,” which is a process that some rocks can go through when exposed to mineral-rich precipitation. While this is typically represented on cliffs and in the center of mountains, it is theoretically possible for this process to occur underground. This is a natural form of fortification, where over time, mineral-rich water penetrates rocks and deposits the microscopic particles of metal within them. This is an extremely gradual process, but it is highly possible that concretion is how these stone spheres got their notorious shape.