Crescent moons atop an Urumqi mosque

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Crescent moons atop of one of the many mosques which line the streets of Urumqi, Xinjinag province, China.

Historically, the crescent was the symbol of the Sassanian Empire of Persia (Iran) and is prominently displayed on the crowns of its rulers. After the Arab conquest of that empire in 651 CE, it was gradually adopted by later Muslim rulers as an established and recognized symbol of power in Western Asia. It was also a symbol of the Ottoman Empire.

Though the crescent was originally a secular symbol of authority for Muslim rulers, it is now often used to symbolize the Islamic faith. However, it should be noted that the crescent was not a symbol used for Islam by Muhammad or any other early Muslim rulers, as the Islamic religion is, in fact, against appointing "Holy Symbols" (so that during the early centuries of Islam, Muslim authorities simply didn't want any geometric symbols to be used to symbolize Islam, in the way that the cross symbolizes Christianity etc.). This is why early Islamic coins were covered with Arabic writing, but contained no visual symbols.