of light 24 whose cross section is of the same order of magnitude as the smallest inclusion to be detected i e 0 03 mm which passes from the laser 12 to the mirror 14 and is reflected from the latter to the precious stone 20 so as to be normal to the base of the stone 20 an ultrasonic transmitter 26 is also provided on a side wall of the container 16 shown in fig1 it is possible e g with a suitable means of driving the plate 18 in the container 16 this means being not shown in the drawing and with a feed device 14 on the mirror 14 to rotate the stone 20 to be examined and at the same time to displace the laser ray 24 in the direction required to produce a spiral scanning trajectory to cover the entire stone the successive spirals being closer than the cross section of the beam i e 0 02 mm this means that the signal which is emitted by the photo sensitive device 22 in the course of this movement and which directly depends on the intensity of the beam of light which has passed through the stone 20 constitutes a kind of spiral development of the volume of the stone.