A lapidary machine adapted to produce cabochon shaped gem stones or the like; the machine having a powered rotary grinding wheel provided with a contoured in cross section annular grinding surface adapted to grind peripheries of cabochon shaped gem stones; a powered rotary dop holding shaft structure adapted rotatably to carry a dop supported gem stone in contact with said annular grinding surface; said shaft structure axially parallel to and laterally movable relative to the rotary axis of said grinding wheel, said shaft structure having a generally oval shaped cam thereon; a normally stationary idler roller having a periphery engagable by said cam and means for adjusting the position of the periphery of said idler roller in directions laterally relative to the rotary axis of said shaft structure in order to progressively cut peripheries of stones down and to produce cabochon shaped stones of various overall sizes.