Carb stones do present a potential contamination issue with their small pores.  That’s why 'Blichmann developed a THREE PIECE design to allow the stone to be completely disassembled for improved sanitation.  And the stem is long enough to fully reach into the main fermentor body, so the bubbles don’t just quickly combine and lose their effectiveness. 

1.5tri clover

This stone can also be used for oxygenating the wort before fermentation!