Abstract: In order to brute a gemstone, the gemstone 6 is rotated about an axis and is ground with a small bruting crown 1 rotating about a parallel axis. In the peripheral working face of the bruting crown 1 are set grinding diamonds or stones 4 each of which subtends an angle .alpha. of at least about 10.degree. at the axis of the bruting crown 1. During working, reciprocatory axial motion is used between the bruting crown 1 and the gemstone 6. In order to set the end position of the bruting crown feed, the image 6' of the gemstone 6 and the image 1' of the bruting crown 1 are projected onto a screen 16. The screen 16 has indicia AB, BC, GH, IC representing the polished stone so that this polished stone can be fitted within the stone image 6'. The magnification of the optical system is changed and the stone is centered so that indicia in the form of bruted girdle diameter lines AB, GH represent the eventual bruted gridle diameter.

Abstract: In a gemstone blocking or polishing machine, a stone holder is supported in a gimbal so that it can pivot about two perpendicular axes, the holder also being capable of rotation about its own axis. The outer ring of the gimbal is in the form of a gear ring which can be indexed, and the holder passes through a slot in a grain ring which controls the orientation of the holder and can be locked to the gear ring by a clutch. In order to change the facet, the gear ring alone is indexed, forcing the holder to twist about its axis. In order to change the grain orientation, the gear ring and the grain ring are indexed synchronously. In order to signal when the facet has been worked to a sufficient depth, an electrical contact is made between the top end of the holder and a horizontal contact bar. The position of the holder is controlled by a spring.