"small, smooth stone," c. 1300, pibel, from Old English papolstan "pebblestone," a word of unknown origin. Perhaps imitative. Some sources compare Latin papula "pustule, pimple, swelling." Historically there is no precise definition based on size other than that it is smaller than a cobble. Related: Pebbly. Pebble-dashing "mortar with pebbles incorporated" is by 1941 (pebble-dash in the same sense is by 1902).
"divination by means of pebbles drawn from a heap," 1727, from Greek psēphos "pebble" (a word of uncertain origin) + -mancy "divination by means of."
"government formed by election by ballot," by 1966, from Greek psēphizein "to vote" (properly "to vote with pebbles"), from psēphos "pebble, small stone," especially as used for counting and calculating (a word of uncertain origin, perhaps related to psammos "sand"), + -cracy "rule or government by."
The common method of voting in Greek cities was by dropping pebbles in different marked urns, and thus the word for "pebble" figures largely in the ancient Greek vocabulary of democracy (e.g. isopsēphos "having an equal vote"). Also a psēphados was "a juggler." Related: Psephocrat; psephocratic.