Well, at that point, my kitchen counter was Formica over plywood, so not much of a radiation source. But I did test my wife's Mexican pottery, and some pretty enamelwork and art glass. Freaked her too ;)
But yes, much granite does contain substantial K-40 (as do we) and some (probably less than 10%) is a substantial source of radon. Also thoron, but it's too short-lived to accumulate. You can buy test kits designed to measure radon production rate from surfaces. The radon gets captured on activated charcoal. You return the kit(s) to the provider, and they send you results.
But yes, much granite does contain substantial K-40 (as do we) and some (probably less than 10%) is a substantial source of radon. Also thoron, but it's too short-lived to accumulate. You can buy test kits designed to measure radon production rate from surfaces. The radon gets captured on activated charcoal. You return the kit(s) to the provider, and they send you results.