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.