Tufa has been used quite often and with good results
Tufa - which is cream, tan or brown. Coralline algea has been reported to grow very well on this rock, however it can contain heavy minerals which can be bad. If you use this rock you should soak it in saltwater for a few weeks and test the water. If yours has been used before then I would still wonder about it. Its the chance you take, but like I said for the most part it has been used with good results.
Thomas