I was doing some research, and from what I found, some "lava rock" is manufactured and is inert. (wont leach). The general consensus from what I saw is most LFS
are selling a safe version of "lava rock" that won't leach, often treated to make it inert. I don't know what "treating" the rock actually is but presumably it makes it safe.
If you're concerned, ask the owner or someone who knows where their lava rock comes from, and can answer your questions. If they will tell you their source, research what they do and use.
If they can't or won't answer your questions to your satisfaction, then I'd use something else, but if you feel good about the answers and info you get from them, then I'd give it a try.