I saw this thread a bit late, but I did drill a natural pearl and here is how I did:
- low speed 1500 tours,
- hollow diamond bit
- water flow through the hollow bit constantly
- count "one, two" down and "three" up the drilling
I did it that way already and there were no crack, it works, but it is better not to drill ayway.
I have seen the most amazing natural pearls at Baseworld by Lan?on and they made a private visit of their collection to me, they do not drill, the setting is intended to "wrap" around the pearl and has to be clever so that not to look like plain wired jewelry.
Good luck