Shaman Stones, Mochi/Moqui Marbles or Thunderballs are roughly spherical iron rich concretions formed in (mostly) Utah sandstone.

Nowadays they are sold in pairs like Boji Stones, though this practice is fairly recent.

I find them to be gently balancing and reassuring.

They are said to be protective and helpful when doing shamanic journeys but I haven’t found that personally.