Oak Leaves?

Oak leaves have been used as insignia by various corps and ranks of the U.S. Navy since the earliest days. This decorative device was probably adopted originally as a symbol of the excellent oaken ships of the United States. In the days of wooden ships, the government preserved its life oak for shipbuilding by placing oak timbers under water for decades.

From: Why is the Colonel Called "Kernal"? The Origin of the Ranks and Rank Insignia Now Used by the United States Armed Forces
Information borrowed Naval Historical Center
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