The sun cross, a cross inside a circle, is one of the oldest and most widely used religious symbols in human history. A powerful symbol of the sun and its diurnal journey across the sky, the sun cross is also associated with the wheel and its invention. Commonly found on rock carvings dating to the prehistoric era, the symbol appears in ancient Egypt, China, Europe, Anatolia, and Mesopotamia. Miniature cast wheels were common votive gifts in the sanctuaries of the sun gods in Northern Greece, Macedonia, and Thrace during the Archaic and Classical periods. Bronze was a natural choice for the metal used, because in addition to its bright, "sun-lit" appearance, bronze was considered a sacred, ‘pure’ material in ancient times (cf. a similar inscribed votive wheel in L.H. Jeffery, The Local Scripts of Archaic Greece [Oxford: the Clarenden Press, 1961], p. 106). Later, such wheel symbols became a common motif on coinage, most prominently seen on the tribal coinage in the Thraco-Macedonian region (e.g. the Edones and Derrones), as well as the coinage of a number of Black Sea settlements, such as Istros.