non-slip granite

Hello, We have installed some shiny black "granite" in an exterior application/Public Place.  It is proving dangerously slippery when wet.  We need to non-slip it.  Is there a way that we can retain the shiny look and have it be non slip?  Will Flaming it do the trick?  Can we do that after it's already installed?  How about acid etching?  We're thinking if nothing else works, we will sandblast in a pattern...? Any help for us non stone experts?  Thank You, Lea Anne