From the Marble Institute link: "Alternatively, if the crushed granite is used to make other building materials, radon emanation can also be a health concern"
Isn't that decomposed granite stuff crushed granite?
Then there is this:
"In the worst case as represented by Crema Bourdeaux at equilibrium is 292 pCi/Lx22÷24000=0.27 pCi/L.
The actual radon dilution factor can be much bigger than what is calculated. Radon from granite countertop can, therefore, only add a very small amount to the house when it is not used such as during the period of vacation. Radon from countertops cannot exceed the maximum equilibrium level because 222Rn decays quickly with a half-life of 3.8235 days."
Then this:
"Conclusion
The testing procedures for radon emanation of granite countertops are designed for normal use of these stones only as countertops. Adapting the 4pCi/L action level as recommended by US EPA as a reference and the house is not in active use, Crema Bourdeaux countertop raises less than 7% of this action level. The second and the third highest radon count stone Tropic Brown and Baltic Brown can add only approximately 1% of this action level. All the other countertops adds only insignificant amount of radon to the house. If normal air exchange rate is applied or the house is actively used, then the radon addition should be much smaller."
That basically tells me not to worry about it. 