A pebble was a small stone created by the action of water.
Trip Tucker described the telepathic granules used by the Xyrillians as a box of pebbles. (ENT: "Unexpected")
While discussing the telekinetic powers of the other Platonians with the USS Enterprise crewmembers. Alexander claimed that he thought it was his mind that couldn't move a pebble - something wrong with him alone, not others like them. (TOS: "Plato's Stepchildren")
According to D'Vana Tendi, beaches on Orion consisted of sharp little pebbles instead of sand. (LD: "A Few Badgeys More")
Moll asked L'ak if he "want(ed) the pebbles or the mountain?" (DIS: "Red Directive")