I Chronicles 20:2 says the following, in reference to David conquering the country of Ammon:
“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” (KJV).
The crown of the king of Ammon was put on David’s head. It weighed a talent of gold. The HarperCollins Study Bible says that’s 75 pounds. How did David wear 75 pounds on his head? That’s really heavy!
The Orthodox Jewish Artscroll commentary lists rabbinic explanations, which are in Babylonian Talmud Avodah Zarah 44a. One is that David did not wear the crown, but it still fit his head. Another is that there was a magnet on the ceiling lifting the crown when David wore it so that it would not be so heavy for him. A third explanation is that the stone in the crown weighed the same as a talent of gold, but the crown itself was lighter. A fourth explanation is that the person who could wear the crown was the one who would be king. It’s sort of like the sword in the stone of Arthurian lore. Rashi says that Joash could wear the crown, and Joash was the one in II Kings 11 who was hidden when the wicked Queen Athaliah reigned in Judah. I can see why Rashi would think that Joash’s right to be king would need to be authenticated after Athaliah was overthrown. And, according to Babylonian Talmud Sanhedrin 21b, the crown did not fit the head of Adonijah, the son of David who was competing with Solomon for the throne.
The Artscroll cites the Zohar, which, in 1:110b, affirms that David’s kingship was to come from Lot. Lot was Abraham’s nephew centuries before. Lot had two sons: Moab and Ammon. David was descended from Moab, through Ruth the Moabitess. But he was legitimized as king through the crown of Ammon.
One thing that I have to give to the “magnet” explanation and the sword-in-the-stone-like explanation: they take seriously the biblical text’s statement that the crown was put on David’s head. It seems to me that other explanations try to skirt this. I read one critical suggestion that crowns were placed on cult statues. Maybe, but the text says that the crown was taken off of the head of the king of Ammon, and put on the head of David.