If you can get appropriately sized granite pebbles for cheap, it should be just fine for AP. The only types of rocks I usually warn against are stuff like limestone/marble that will mess with pH. Otherwise, I use the cheapest gravel I can get, around here that is the 1/2" (and smaller) brown river rock which is mostly quartz based rounded pebbles. Granite costs more down here in FL.
So I ran a ton of 12 x 12 granite samples up to Boulder Colorado to a man that had an XRF gun, looks like a hand held radar gun, spits out a list of heavy metals present in what you point it at. Yikes, lead, arsenic, cadmium, chromium, lots of uranium, and dozens more heavy metals. Everything except beryllium. Finding beryllium was the Holy Grail but my budget was limited to a hundred scans so we didn't find that one.
Amounts varied of course from small trace amounts to ore quality, as much as 1% of some heavy metals, much more for common metals like lead.
The reason natural lava rock is so dangerous is that it may contain heavy metals that can leach into your tank. Am I right?
in principle yes BUT .... it isn't heavy metal which is that dangerous in the long therm ... it is sulfur/sodium-sulfur and something like that; materials which solve in sea water very fast and poison the water and ALL animals.