First, not all granite contains radon. Green is relative depending on your definition. I have see plenty of cases where people will point to the mercury released in the manufacture of concrete (and/or other items) to say it isn't green.
Personally, I have no issues with granite, concrete or even many synthetic countertops assuming it is the right material for the kitchen and how the homeowner normally cooks.
One of my favorite materials is countertops made from recycled glass which use an epoxy resin and give a great look.
For more information:
Read Eric Corey Freed's article "5 Gorgeous Green Countertop Materials".
Also, read Sean Lintow's blog post "Granite - Maintenance, Upkeep and a Geiger Counter?"