Planted Tank Enthusiast
Join Date: May 2019
Location: Palm Springs, CA
Just came across this thread. The Mojave high desert has lots of cinder cones that are basically low mountains of lava rock. There are some along the 247 near Landers, Pipes Canyon, etc. Likely there are some closer to you up in Apple Valley as well. I've seen black lava rocks & boulders just bulldozed along the side of the road, so if you can find the right area, they'd be there for the picking....
I've gotten scavenged small lava rocks for attaching java fern & moss to. They make good pumice stones too . And while you're poking around the desert, you'll likely come across lots of cholla like they sell at fancy LFS & aquascaping sites.