You're probably tired of hearing from me but...
The mineral make up of lava rock varies due to its magma creation. Metal content is most notably the concern. Some report problems right away while others are ok for a while before things start to go south.
Here is your wiki-answer...
http://en.wikipedia.org/wiki/Basalt
Aside from that, the structure of the rock leads to the collection of A LOT of detritus which leads to either nutrient issues of a whole to of cleaning.
Bottom line for me is that we spend a lot of time and money on our tanks and using lava rock is simply not worth the risk. I know you have the rock already but dry aragonite is cheap, proven and safe.