Almost impossible to say what type they are as they are pretty well covered with algae. But then if you want that look and mbuna love the alkaline from limestone, I would go for the easy way. Either pick it up or if less inclined to do that, it is easy to find much the same shape and look at rock places who sell landscape rocks. They will have a wide range of shapes, sizes and type from which to choose. Reasonable prices as well. It is highly unlikely you will find any that give your water much trouble that will not be just what your type fish will like.