although i don't know what the rocks are per se, but i there are tips to see if the rock is safe for fish. 2 of the most well known tips are to
a) make sure that the rock has no shiny silver colored minerals at all as those can leech into the water, harming your fish
b) pour some vinegar on it to see if it bubbles and fizzes. if it does, the rock will definitely raise your pH
those are the only 2 methods i know of...hope they helped