And David took the crown of their king from his head. He found that it weighed a talent of gold, and in it was a precious stone. And it was placed on David’s head. And he brought out the spoil of the city, a very great amount.
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.
Commenting on 1 Chronicles 20:1-3
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.
And David took the crown of their king from off his head, and found it to weigh a talent of gold, and [there were] precious stones in it; and it was set upon David's head: and he brought also exceeding much spoil out of the city. (b) Which mounts about the value of seven thousand and seventy crowns, which is about 60 pound weight.