volcanic rock

I was about to transplant a new plant into it's 5 gal, but I'm out of the rocks I was putting at the bottom of my buckets to keep the soil from blocking the drain holes. What I do have is a bunch of little volcanic rocks, you know the porous kind with all the little holes in them. On the surface I would think they might be perfect for the job, but then I started thinking about where they came from and I'm wondering if they might turn the lower soil really acidic.
I was wondering if by chance anyone knows anything about this. I've used them before in a regular garden with cactus but never in a application like this.

use em to absorb odors

they work great.