most lava rocks are actually igneous rocks formed during volcanic eruptions. the difference in lava rock and live rock is the chemical content. live rock is primarily calcium carbonate. this is alkaline due to the metal calcium. but i will not go too much into chemistry here im sure none of u are interested haha.
lava rock (igneous rocks) have a high content of silicon dioxide. as silicon is a non metal, it forms acidic oxides. silicon dioxide is acidic and thus, lava rock is acidic as compared live rock which is alkaline. this is why most brackish water set up employ the use of lava rock to make the water more acidic.
there are a few kinds of igneous rocks. acidic, basic, alkalic and some other terms. the basic and alkalic ones contains alkali which is similar to calcium carbonate rock. however the difference is that the alkali chemical in the rock is not calcium. it is potassium or sodium. these 2 metals are also alkali, like calcium, but it will not help in calcification of water like what liverock does.
ok. main thing is, use liverock haha (: i think with ur lava rock, also can. just that live rock will be better. any rock experts here can please correct me if im wrong. just trying to help (: