Atlas stones are the "standard" stones, but they're concrete spheres, not rubber-coated. Lifters use pine resin, "tacky", on their hands to make them easier to grab. Or less hard to grab, at least.
(Note that the linked article talks about not using tacky on natural stones.)
I was thinking something irregular (accidentally typed symmetric in original comment) like in stone lifting, but with some rock climbing handles. Surprised something like that does not exist. Seems like it would be a neat training implement, especially if difficulty can be standardized. I like the idea of picking up stones, just in more commercially accessible and clean / less pain in the ass way. My original thought was something like Rogues rubber atlas vs traditional concrete cast atlas but stone shapped instead of ball shapped.
(Note that the linked article talks about not using tacky on natural stones.)