I think there is way too much time spent on what things we put in the tank might be. We want to have a name so we can look on a list and see if it is listed as safe/ hazardous. What we don't like is the feeling of not knowing. But that is where things get tricky. There is no "standard" answer which is correct in all cases. Much depends on what we want from the decor we put in the tank. If we have hard water and want it to drift softer, adding some driftwood may do it. If we have hard water and want soft, don't add a huge chunk of limestone!
Meanwhile it is much simpler to go with what we like and find out what happens. Since rocks are pretty slow to degrade, it is unlikely any rock is going to change your water overnight if it is not already way off the chart acidic.