Then the men of Israel said to Gideon, “Rule over us, you and your son and your grandson also, for you have saved us from the hand of Midian.” Gideon said to them, “I will not rule over you, and my son will not rule over you; the LORD will rule over you.” And Gideon said to them, “Let me make a request of you: every one of you give me the earrings from his spoil.” (For they had golden earrings, because they were Ishmaelites.) And they answered, “We will willingly give them.” And they spread a cloak, and every man threw in it the earrings of his spoil. And the weight of the golden earrings that he requested was 1,700 shekels of gold, besides the crescent ornaments and the pendants and the purple garments worn by the kings of Midian, and besides the collars that were around the necks of their camels. And Gideon made an ephod of it and put it in his city, in Ophrah. And all Israel whored after it there, and it became a snare to Gideon and to his family. So Midian was subdued before the people of Israel, and they raised their heads no more. And the land had rest forty years in the days of Gideon.
(Judges 8:22-28, ESV)
Within the context of the story, the author is setting up the reader to understand Gideon's fall, with Israel thinking to make him king, Gideon resisting the temptation in part, but yielding to the temptation to an opulent use of the spoil (verging on if not attaining idolatry) from the war with the Midianites. To do so, he orders the golden earrings and crescents to be collected. The story almost lends itself to be saying, "look how many Midianites were killed based on the earrings and crescents."
The meaning of the crescents are straightforward: they were probably moon worshipers. I'm wondering what it is about the earrings that made them significant. Why would it be considered tautological that Ishmaelites wear golden earrings?