Be careful boiling it, it might crack, depending upon the piece. I would skip that and just scrub it with a brush, hose it off with the hose on its strongest setting, and then dunk it in bleach water, about 5:1 (water to bleach) or even weaker would be fine. Then you will need to soak it in water with lots of dechlor, in case there are any cracks in the stone that are holding bleach. You could also sit it out in the sun for a day to be sure the bleach is gone.