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Since the main issue is the organic material trapped inside the rock itself, I would say your best bet may be to soak the rock in fresh water and test it every few days for ammonia, nitrite and nitrate the same way you would watch to see how the cycle is coming along in your tank. Once the rock is fully cured you should see no more ammonia or nitrite and your nitrates will go up. By then you shouldn't smell anything foul anymore, just an "earthy" smell.
Even if you give it a bath in bleach, it will kill bacteria and mold if there is any, but it will not remove the organics. So you have to decide which method makes the most sense to you (Boiling water bath, bleach, peroxide as your initial attack to kill any bacteria and/or mold then soak it and watch for the levels to spike telling you whether there is still organics decomposing or not.)
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