The circle is roundly regarded as the single most powerful symbol of all. It is Infinity and the Universe; Alpha and Omega; Beginning and End; Creation and Destruction; Destiny, reincarnation, and Fortune.
The circle stands for the sun and moon, the zodiac, power and protection, the crown of kings, and the tiara of popes (most often depicted as a halo). It is the only single-line diagram that magi use to mark the bounds of their sacred -- and thus safe -- space. In this way, bracelets and rings gain greater significance, especially when and where they are worn and with what inscribed on them.
The circle can also represent the eye and, by extension, an omnipotent god. Divine Energy, it is said, always flows in a circular form, giving rise the concentric rings of the Heavens and Hell.
Pagans use the circular form of a snake eating its own tail -- a symbol of the infinite depth of Wisdom later recast by the Church as the ultimate symbol of Evil.