How the army of the Ammonites and their allies was routed in the field we read in the foregoing chapters. Here we have the destruction of Rabbah, the metropolis of their kingdom (Ch1 20:1), the putting of their king's crown upon David's head (Ch1 20:2), and the great severity that was used towards the people, Ch1 20:3. Of this we had a more full account in 2 Sa. 11, 12, and cannot but remember it by this sad token, that while Joab was besieging Rabbah David fell into that great sin in the matter of Uriah. But it is observable that, though the rest of the story is repeated, that is not: a hint only is given of it, in those words which lie here in a parenthesis - But David tarried at Jerusalem. If he had been abroad with his army, he would have been out of the way of that temptation; but, indulging his ease, he fell into uncleanness. Now, as the relating of the sin David fell into is an instance of the impartiality and fidelity of the sacred writers, so the avoiding of the repetition of it here, when there was a fair occasion given to speak of it again, is designed to teach us that, though there may be a just occasion to speak of the faults and miscarriages of others, yet we should not take delight in the repetition of them. That should always be looked upon as an unpleasing subject which, though sometimes one cannot help falling upon, yet one would not choose to dwell upon, any more than we should love to rake in a dunghill. The persons, or actions, we can say no good of, we had best say nothing of.

David took the crown of their king - off his head - See Sa2 12:30.
Precious stones in it - The Targum says, "And there was set in it a precious stone, worth a talent of gold; this was that magnetic stone that supported the woven gold in the air." What does he mean?

See Gill on Ch1 20:1, Sa2 11:1, Ch1 20:2, Sa2 12:30, Sa2 21:15, and Ch1 21:1.

Next: 1 Chronicles Chapter 21

David took the crown of their king . . ., and found it to weigh a talent of gold--equal to one hundred twenty-five pounds. Some think that Malcom, rendered in our version "their king," should be taken as a proper name, Milcom or Molech, the Ammonite idol, which, of course, might bear a heavy weight. But, like many other state crowns of Eastern kings, the crown got at Rabbah was not worn on the head, but suspended by chains of gold above the throne.

precious stones--Hebrew, a "stone," or cluster of precious stones, which was set on David's head.