rocking-stone

CIDE DICTIONARY

rocking-stone, n. 

 A stone, often of great size and weight, resting upon another stone, and so exactly poised that it can be rocked, or slightly moved, with but little force.  [1913 Webster]

ROGET THESAURUS

rocking-stone

Oscillation

N oscillation, vibration, libration, motion of a pendulum, nutation, undulation, pulsation, pulse, alternation, coming and going, ebb and flow, flux and reflux, ups and down, fluctuation, vacillation, wave, vibratiuncle, swing, beat, shake, wag, seesaw, dance, lurch, dodge, logan, loggan, rocking-stone, vibroscope, oscillating, oscillatory, undulatory, pulsatory, libratory, rectilinear, vibratory, vibratile, pendulous, to and fro, up and down, backwards and forwards, hither and yon, seesaw, zigzag, wibble-wabble, in and out, from side to side, like buckets in a well.