Lava, Tufa Rock

[Black Phantom]

Do not put any lava rock in your aquarium. The chances of some sort of iron or metal being in the rock are quite high. This will cause you to have all sorts of algae outbreaks. There are plenty of DIY sites on how to make your own rock for pennies a pound. It just takes a little time and effort. It makes a great base rock. Then add a little live rock to top it off.

[SeaShell]

Here's a great link for creating your own aragocrete rocks:

http://www.athiel.com/lib3/arago.htm