Aluminum oxide pre-polish.
You can use this instead of a 600F grit, or in addition to it. Your results will only get better. This is mixed grade, averaging around 6000 grit, so it does an excellent job prepping for the final shine.
Word of caution: take extra time to make sure you have no cracks or crevices in your rocks. It will embed itself quite firmly and can be difficult to remove!