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I only recently learnt that the crescent moon (and/or star) wasn't actually a Muslim symbol until fairly recently, and that's because of the Ottomans' adoption of secular moon-and-star symbolism (from the Byzantines and/or Turkic peoples), and subsequent reinterpretation in the wake of their dismantling.
Given that, what other symbols (as antithetical as icons are to Islam) could have arisen? I'm favoring Zulfiqar, but are there any abstract symbols which would lend themselves well to the proselytising of Islam?