Jamieson, Fausset and Brown on 2 Samuel 12:30

30. he took their king's crown from off his head--While the treasures of the city were given as plunder to his soldiers, David reserved to himself the crown, which was of rarest value. Its great weight makes it probable that it was like many ancient crowns, not worn, but suspended over the head, or fixed on a canopy on the top of the throne.

the precious stones--Hebrew, "stone"; was a round ball composed of pearls and other jewels, which was in the crown, and probably taken out of it to be inserted in David's own crown.