Lava Rock is a naturally occurring, paramagnetic, mineral rich volcanic cinder that has a porous structure which makes the material lightweight and highly moisture retentive.
Incorporate into your Bonsai soil mix to increase drainage (aeration). It will absorb up to 40% of its weight in water. Small lava gravel (1/4 to 3/8 inch) is the best component because it has very "sharp", irregular surfaces, which help to produce a well branched root system.
This soil additive is considered by many to be an excellent source of energy for the soil.
Think about the weather your plants will experience. Get soil advice from a bonsai grower who lives in the area.