You've probably never heard of TENORM, which stands for technologically enhanced naturally occurring radioactive material. Everything on earth emits some amount of radiation, even humans. Some rocks contain massive amounts of radiation, granite being one of the worst. The radiation can be released in larger amounts if rock is crushed, cut or otherwise altered by humans. Before you install granite countertops in your home, invest in an inexpensive handheld geiger counter and take it with you to the supplier you plan to purchase your new countertops from. Run the device over the countertops to determine the amount of radiation they release before purchasing your new tops. It's much better to be safe than sorry.
Karen