There is something extremely odd about this tombstone. It doesn't feel like a regular tombstone... it feels alive.
Gurvari creatures possess tombstones, funnily enough! Maybe they are actual ghosts of something or someone once-living, maybe of a specific rock-elemental creature that died and that's why it's possessing the tombstone?
Or maybe it's an accidental possession. Or this possession happens only under very specific conditions. Alternatively, the tombstones could've been blank stone erected solely for the purpose of providing a vessel for summoning these creatures via magic or specific ritualistic items.