You can use Windex on granite but not too often.  Granite countertops and floors have a sealant that keeps them all nice and shiny as well as stain resistant.  Windex will not only weaken the sealant but also make it look dull.  If you want to use Windex on granite surfaces, use the Windex Multi-Surface Disinfectant Cleaner as it doesn’t contain ammonia.