Take a good look at it, making sure to check cracks and crevices. It may have some dying critters like clams or sponges that will continue to decay in your tank for a long time. You could take a powerful powerhead and blow them off underwater, use a couple of different buckets to dip and swish until the rock looks good. It's tuff to say without seeing the rock. Some rock has a lot of live like sponges, algae, coral ect and some has very little. If it looks clean you are probably fine. Either way I would at least put it in some buckets with powerheads and a heater and test for ammonia/nitrite/nitrate for a couple of days to be safe.