Lava rock is an excellent base for an inorganic rock mix. This lava rock has good water retention with excellent aeration. Because it is inorganic, the mix will not break down and orchids that resent repotting can be left in their mix longer. This large size is suitable as a base media and for matured orchids.
You can learn more at http://staugorchidsociety.org/PDF/201002Tips-GrowingPaphsinLavaRock.pdf
This lava rock is also suitable for your aquarium decoration.