The tufa rocks I did come across at local pet shops, do look as if it could easily break down over time. The rocks that I have access to chip off easily when bumped into another rock. If there are minerals in the water and soils being used, I would think that they would build up in the pores and possibly cause the rock to break down over time.
The tufa rock that Tamlin has posted above, does not look like the tufa rock available to me. Tamlin's rock has fewer pores and looks smooth. The rocks available to me are very porous and ruff. It could be that the rock Tamlin has posted is weathered, where the rocks I have access to have not been exposed.
I still plan to mount a ping on the rock I did find.