Under the cautionary heading of "Don't try this at home" BUT! here's how I do it: first, you have to keep the stone and drill bit (pointy end) under water! (This keeps both the stone and the diamond bit cool) and it takes a long time! You can't push/force the point into the stone; just hold it in place and "let the drill do the work" . . . you didn't say whether you're trying to enlarge an already existing hole or if you are starting from scratch. If it's the latter, you should start the hole with a different shaped bit, one that is (counter-intuitively) rounder, to make a little divit for your drilling bit to nestle into. You need two of those little divits, on either side of the stone, and drill half way (outside to center X2) . . . clear? Of course be careful, but I've drilled bizillions of 4mm tourmaline (much harder than agate) without any trouble; just takes time!! Good luck! Jan