While most experts agree that only a small percentage of granite in homes today poses any health risk, the current debate centers on identifying granite that might emit radiation and determining under what circumstances a danger occurs.

"The vast majority of granites in the vast majority of houses are not going to be a problem," said Daniel Steck, a physics professor at St. John's University in Collegeville, Minn., and head of the Minnesota Radon Project. Steck has tested about 250 granites. Of those, none emitted enough radiation to be a concern, he said.

But a small minority, about 5%, appeared to contribute enough radon to pose "a mild problem" when used in large quantities or in a small, tightly sealed home.

You can't spot a hot slab by its looks, color or its name. Granite names are variable, and stones with the same name can have different geological content, Steck said. Right now, "There's no good way to identify that 5%."