Abstract: An apparatus for mounting gemstones in prong type settings by cutting a seat in the form of an inset edge in the ends of the prongs and swaging the remaining prong material over a stone girdle, comprising a vertical support frame holding a tool driver such as a rotating chuck assembly slidably mounted thereon for sliding between first and second predetermined limits of travel. A rotary cutting tool is coupled to the tool driver for being rotated and removing predetermined portions of the prong ends. A cylindrical mandrel or vise clamp holder is mounted on a work platform attached to the frame and positions the setting under the cutting tool. The setting is held in a fixed vertical but self adjusting lateral or horizontal position which provides automatic centering of the mounting operation. A reciprocating element moves the tool driver up or down on the vertical frame with a stop being employed to control spacing between the cutting tool and the setting.