I use calciferous rock (petrified coral rock, aka "base rock") for my African tanks because it buffers the water. This is the rock that a
SW dealer might use to grow live rock, and it is true - it is expensive because the rock is crawling with all kinds of critters and it has to be kept alive during shipping, just like fish.
You can use base rock in any tank where you want to buffer the water or raise the pH, and often a brackish setup will benefit from that, but it still would not support live rock or live sand. I got some base rock from a
SW LFS near me for $1.00 per pound, which I was pretty happy with.