A decorative lava rock mulch will not hurt your rubber tree plant.

I just saw this, sorry it's been a year!!

I did this exact thing, using red lava rocks. After a year or two, though, they had all turned brown and looked very ugly! Worse, my cats took it as a challenge and tried to use it as a litter box. And my beagle thought it would be awesome to pull the lava rocks out and try to eat them!!

I ended up having to dig them all out. So I would not recommend doing it.

Use red glass beads from the craft store instead. They won't turn brown. I don't have a beagle and my cats don't do that.

Hahaha, I realize that my beagle issue is probably kinda unique ;-) But it's something to know, anyway.

That dog was a nut, if I let him in the back yard unsupervised he thought that rocks and acorns were awesome treats! LOL His favorite toy as a pup was an empty milk jug, he would tear it to pieces for DAYS!