Ouch! What a shock that must have been. I have to second the warning about never drilling "calcereous concretion" varieties of pearls. I have a very sad but true cautionary tale of such matters: Many years ago, when I was much younger and inexperienced in such things and much more ignorant even than I am now, a man brought a Tridachna pearl to the store where I was working. It had been a gift to him by a native polynesian islander. Never having seen or even heard of such a thing, we arrived at a design for it that involved me drilling it for a post mounting. Yes, it developed a terrible crack that, if I remember correctly, went most or all of the way around it, though it stayed intact.
Needless to say, all involved were in a state of shock and most utter dismay. The customer involved was a reasonable person, and because we were quite willing to do almost anything we could to compensate him, we arrived at a reasonable conclusion. However, we all realized, sadly, that nothing could really make it "right" again. I now approach the unknown with
much greater caution.
Don't ever drill these concretions!
J. Marcus
http://flyrodjewelry.com/home.html