After mining block below worldgen-generated rock, that rock drops. It’s material is stone (depends on dimension; in Atum it was limestone, I think). Problem is, I previously “scanned” it with magnifying glass, and it was some mineral or metal. Several such rocks stack in inventory. Easy to reproduce (always or very often). Singleplayer world. Version: 6.15.08.
Aaaaand it is fixed. ^^