FWIW, the larger pieces of red lava rock I'm using came from a local landscape supply. They had it stored outside in the yard in about a dozen gaylords (large bulk boxes made out of wood or cardboard, with a pallet on the bottom)
This was in a parking area where there was a lot of truck traffic (dump trucks, cars, pickups, etc.) ... given that it was in the winter, I suspect that everything I picked up had a good coating of salt and/or deicer on it.
What I did before I used them was rinse them thoroughly, then take a large, deep pot we use for canning (the boiling water-bath method, with mason jars) and boil the rocks for 15-20 minutes ... and then rinse them again very thoroughly.
The small lava rock that I've used in filters, I picked up from Lowes or Home Depot and which came bagged, I just rinsed off real well before using it.
I also used that big pot to boil a piece of mopani wood I put in the BNP tank, to try and leach out the tannins ... probably spent the better part of a day doing that. Seemed to have worked pretty well as I've not noticed any appreciable discoloration of the water ... but the tank itself is pretty dark (Black Diamond blasting media as substrate, and a black-painted back)
FWIW, we do have an iron/sediment filter on our house water system - unfortunately, with the way it is plumbed in, it is not really possible to run the well water through it without also running it through the water softner.
One of these days (Ha !) I might get around to re-plumbing it so it can be used to filter the hard water as well the soft.