While granite and other decorative stone can contain some trace natural radioactivity, granite countertops are not considered to be a key contributor to radiation in the average home.
Radioactive minerals in the granite can decay into radon – a colorless, odorless, radioactive gas – which may be released from the surface of the granite over time. However, since the granite is not porous and exposed surfaces are treated with sealant, it is harder for the radon escape from the granite than from more porous stones such as sandstone.
While radon gas and radiation emission levels attributable to granite are not typically high, there are simply too many variables to generalize about the emissions from granite countertops. However, high levels of radon in the home (from all possible sources) is a health concern because it can cause lung cancer. The Surgeon General and the EPA recommend testing for radon and reducing radon in homes that have high levels.