Looks to me to be some sort of lava rock. I'd not hesitate to try it although I'd position it in such a way that it was used as the base rather than the crown of the display. Just for aesthetic reasons.
Yes, it's probably safe. And is, probably aggregate picked up in and around the quarry boundaries.
But, a little prudence can certainly go a long way in sorting these things out, hmmmm?
So, what would I do? WWAD?.....
- Place one (?) of the affected rocks in a bucket or tub. Let it soak for a couple days. Then vigorously scrub and scrape at the aggregate area. is it rock? Or, REALLY hardened clay/caliche deposits on your Texas Holey Rock? If that stuff just doesn't come off of the rock after that? Good! It belongs there.
I don't see any reason to hesitate. It doesn't appear to have any metallic substances. It almost looks like a very hardened clay. I know I'm a few days late to the post but letting it soak in a water change bucket for a day or two might be a wise idea if you're uncertain. If nothing else, it'll clean the surface before you add it into the tank itself.