I might be getting ahead of myself here. I don't have a scientific answer for you. However, marble seems to me to be more inert; having a more "glass-like", none pourous quality. Crushed limestone gravel is more "rough" textured, absorbs water and it will slowly over time, disolve into solution; providing a pH buffering aspect as well as calcium. I have no idea about whether or not dolomite gravel would be good or not. My feeling is that since the plants are growing on limestone outcroppings in nature, why not just use limestone in the potting mix, instead of a substitute? It's cheap and readily available (except apparently, in NY city). Using something else that may make calcium more "available" doesn't necessarily mean that it's better. You could end up with a calcium toxicity problem. I believe in K.I.S.S. If calcium works in nature and it's cheap and easily available, then, why risk mucking things up by experimenting with other additives.