My feeling is there is no reason to panic. You have already tested your house for radon, right? Barring buying a Geiger counter or hiring an expert, most people won't be able to determine where they stand. The good news is there are ways to mitigate potential problems. And these are almost always good things to do whether you have granite or not!
- Ventilate your house as often as possible with outside air.
- Get a home radon test kit.
- Maintain your countertops.