Oh man, it's been a while since I've had geology. But the only rock that I can remember having a sandy texture is sand-stone. Sandstone itself can be a conglomeration of several types of minerals that are cemented together. It tends to be composed of clay, calcite, and silica, all of which are fairly chemically inert. I believe the "vinegar test" really shows if the rock contains calcium carbonate, Ca CO3, which would up your KH. If you want to test for that use a HCl solution, I believe someone said that one of the API tests was a HCl solution. But I believe that most rocks that contain Ca CO3 are white in color, I also believe that sandstone, if that's what this is, contains almost exclusively sand, Silica Dioxide, which makes it just like putting sand in your tank.
They do look really cool though, especially the one on the right with all those fractures and such.