Crescent

adopted late, and not from scripture

Unicode: U+263D first quarter moon
Meaning: The waxing or waning moon; and, since the Ottoman period, an emblem of Islam.
History: The most consequential fact about this symbol is a negative one: it is not Quranic, and its association with Islam is a late political inheritance rather than a religious prescription. The crescent was in use in the eastern Mediterranean long before, and reached its modern meaning through Ottoman state usage and the flags that followed. Older careers: the Mesopotamian moon god, Artemis and Diana, and Marian iconography.
Variants: Orientation varies by flag and by hemisphere; the paired star is a separate element with its own history.
Related: ☉ Sun; 星 the star forms; the Moon archetype.
Appears: National flags across North Africa, the Middle East and Asia; the Red Crescent; astronomical notation.
Evolution: Lunar sign → dynastic emblem → the marker of a faith that never asked for it.

Sources · 3

documented The absence of the crescent from the Quran, and its adoption through Ottoman state and military usage.
documented Lunar deities and crescent iconography in Mesopotamian and Greco-Roman practice, long predating Islam.
traditional The Byzantine crescent of Constantinople as the direct antecedent of the Ottoman use. Frequently told, and the transmission is not clean.