I found this on Charmouth Beach in very muddy sludge. It's about 30 cm long and 18cm wide with mud coloured rock on the outside and what look like brown crystals inside in a strange pattern at the top. It's very heavy
1. Limestone, like most rocks, can have veins - minerals filling cracks.
Sometime these are obvious, because the vein and rock are different colours and/or the veins are wide.
But veins occur down to very small scales, and if you have a narrow vein that happens to be filled with mineral of a similar colour to the rock, it can be very difficult to see.
2. Veins, and/or the cracks that preceded/accompanied them can range in length from millimetres to metres. The ends of solitary veins just peter out (with multiple veins, they may end by meeting another one).
3. The mineral comprising the veins is deposited from fluids that permeated the rock - hence potentially affecting its chemistry/mineralogy. It may be that the mineral in some veins was transported only a few cm, causing depletion of chemicals from the nearby rock, which may make that depleted part of the rock weaker. In that sense, there is a concretionary process at work, which may result in much later erosion producing cobbles that have a vein at their core.
(There are other types of vein, associated with igneous and metamorphic processes, whose minerals come from much further away.)
Your specimen probably came from a volume of brownish limestone that had subtle veining, and your cobble happens to have split along one of these veins - exposing the calcite (as Shompy writes).