I do have over a dozen pieces of Mopani that I got new over the years. I like it because it is pretty dense wood and does not rot as fast as Manzanita, Malaysian driftwood, or spiderwood. It also smells like tea when boiled ☺
I also happen to like the black water look. But back to OP, Purigen is also re-newable.
As everything else, rocks or wood, it's all a matter of personal taste. I would go to your local landscaping / rock yard and check what they have. You might like the looks of Lava rock and it comes in multiple colors, from dark black, to rusty red, to orange red.
Enjoy the hunt.
Courtesy * Integrity * Perseverance * Indominable Spirit * and Self Control
Tenets of TKD