Albert Barnes Commentary 2 Samuel 12:30

"And he took the crown of their king from off his head; and the weight thereof was a talent of gold, and [in it were] precious stones; and it was set on David`s head. And he brought forth the spoil of the city, exceeding much." — 2 Samuel 12:30 (ASV)

Their king’s crown - The word rendered “their king” (Malcham) is also the name of the national idol of the Ammonites (Jeremiah 49:1; Jeremiah 49:3, margin; Amos 1:15; Zephaniah 1:5). Moreover, the weight of the crown, which is calculated to be 100 or 125 pounds, is far too great for a man to wear. On the whole, it seems most probable that the idol Malcam is meant here.