If you break off a piece, you will see that the black color is present near the surface and it grades to gray inside. Second, origin – it is being dissolved largely by algae, bacteria and fungi so it is often called "biokarst". These tiny (microscopic) organisms bore into the rock and dissolve the calcite crystals. They are most dense within a centimeter or so at the surface, causing the black color. Why this results in the characteristic macro-texture is not fully understood, but you always see the two elements together so they must be linked somehow. It is clear that proximity to the marine environment is key. Maybe the splash, spray and mist from the waves breaking provide just enough moisture to sustain the algae. So, when you travel around the Caribbean, look at the rocky shoreline, if it is limestone, biokarst may be well developed. The most famous occurrence is at a place called Hell on Grand Cayman." References External links Landforms of the Cayman Islands Grand Cayman