Dog Rocks - do they work, are they dangerous.

Dog Rocks are supposed to stop your dog's wee from turning grass yellow.  You put them in your dog's water.  Does anyone know of a link to a proper scientific site where these rocks have been tested for a; effectiveness and b; safety.  Plenty of sites say they work but most of those sites are selling the rocks so "they would say that wouldn't they"?  Some people also say the sulphur in the rocks can cause kidney and bladder problems for dogs.  I recently bought some dog rocks but now I'm having second thoughts and would like to see some results based on research, as opposed to unfounded opinion, before I continue using them on my dog.  Any advice would be appreciated.