Diamonds CAN scratch and they CAN crack! I had a full channel band from Tiffany's as my wedding band but, I kept messing it up and cracking stones! I do A LOT of work with my hands and I had asked them specifically if this would be a problem and was assured it wouldn't be...well, after taking it back twice, we just swapped it out. My DH and I got a slightly larger stone (hey, I was losing HALF!) to make me feel better. The manager at Tiffany's said that it IS possible to crack, chip, or scratch a diamond! The settings can be tight and the pressure from the stress could cause the crack--it's no one's fault! It can just happen! YOUR ring has a warranty and your stone should be replaced with some possible minor costs to you. It is likely that the cut of the diamond was compromised and the stress/pressure of the setting cracked it after a minor whack on the hand.
If you word it as such...that you aren't placing blame, that accidents happen, that you know the merchant didn't cheat you (I'm not saying either way! I'm just thinking of ways to make the merchant more inclined to assist you), that after careful researching, you've learned that these things happen...you'd like the stone replaced. If they STILL won't help--take 'em to the cleaners!
Good luck!