It is difficult to ID the rock just by looking. The most concern is that it may increase your ph and hardness, i.e., avoid limestone or calcareous rocks. The easiest way to tell is to use a pocket knife, if you can scratch the rock, it may be limestone. if you can’t, it is definitely not and safe to use. If you can scratch it, then do an acid test, vinegar will be OK, Drop some vinegar on the rock, if it bubbles up, even slow, small bubbles, , it is limestone or dolomite. If not, it’s safe to use.