Lava rocks

I use it for the substrate in my crab/ shrimp tank which has a under gravel filter.
I also use as a substrate cap in my dirt tank.

I recommend rinsing it very well though because it has a lot of dust in it/ on it when it comes out of the bag.

Edit: I don't recommend under gravel filters though. They are too hard to clean properly. Food and muck has gotten under mine and now I have a planaria problem that I can't starve out.

You can also get smaller lava rock in places that sell rock for landscaping. (well, around here, anyway)
And yes, lava rock is safe for aquariums, but can be quite messy to initially clean up.

also check out if lava rocks raise ph, hardness anything before hand