Any slow setting epoxy will work. I use system3 on most of my work. Go with a 30 minute epoxy, use a liberal amount (put it on like peanut butter), 2 pistol clamps with padded jaws. You will get some squeeze out when you clamp them. Take some painters tape and run it around the bottom stone. That way the squeeze out will run down the tape and not the stone. Then after an hour un-clamp, remove the tape and let it set for at least a day to fully cure.