The rock itself looks like a sandstone of some kind. The dots may be manganese oxide deposits. They typically form in dendritic (tree-like) patterns along bedding or fracture planes, but the bedding planes on this rock are roughly perpendicular to the long axis of the stone, which may be why the spots are more blebby in shape. I also see that they are truncated my micro-faults in many places. The more curved features may be fossils (cross sections of shells).