Abstract: People usually use fluorescent desk lamp to observe and select diamonds with better cut. However, when viewing diamonds from different light angles, we will find it difficult to choose diamonds with better cut because of different brightness. In order to find out the characteristics of diamond under the desk lamp, the author previously used simulation software to study the images of the diamonds under the simulation "desk lamp" in 2017. It was concluded that when the incident angle of the desk lamp is between 20° and 25°, the diamond with better cut should be selected.Since there is a gap between the optical effect of the simulated diamond and the actual diamond under the lamp, whether the conclusion of simulation study can accord with the actual situation needs to be verified by actual observation.The author selected round diamonds of different cutting levels and invited a large number of observers to observe them with naked eyes under the real desk lamp and finally obtained the following research results.In this paper, 48 real diamonds with different crown and pavilion angles were used for further research under the real desk lamp. The crown angle is from 27°-40° (each time changing 2°), and the pavilion angle is from 38°-43°(each time changing 1°). The cut combination number is 48(8×6). They were discharged on a grooved white plate and subjected for visual observation. Specifically, the white plate is placed vertically under the lamp, and then the plate is tilted at different angles (15°-30°, each time changing 2.5°) using a square plate. The direction observing sight is vertical to the white board. Observation results show that when the tilting angle of the white plate is within 20° - 25°, the good cut diamonds can be selected since the bright upper part and uniform overall brightness. Almost all diamonds become dark when the tilting angle of the white plate is above 25°. When the tilting angle of the white plate is less than 20°, the bottom half of the diamonds with small crown and small pavilion angle shows brighter. The experiment verified that the results of the simulation study were generally consistent with the actual observations.Due to the differences in observation methods and habits of different observers, the author asked 100 observers in jewelry industry to sway diamonds randomly under the desk lamp and evaluate the diamonds comprehensively. In this way, the diamond viewing angle and the distance and angle between each observer and diamond light are different. It leads to the difference in evaluation results.When the tilt angle of white plate is fixed 25° in the laboratory, most observers can pick out the better cut diamonds and exclude the diamonds with larger or smaller angles.In order to display the observation effect to readers, a camera is used to capture photos after adjusting to reasonable shooting parameters, which can replace the human eye observation to a certain extent. The diamonds in the red frame range are VG or EX grade. The research in this paper shows that the results vary greatly when the diamonds are observed in different angles under the desk lamp. When the tilting angle of the white plate is controlled within 20°-25°, and the sight direction is vertical to the plate with the forehead placed against the lampshade, it is easier to choose good cut diamonds by the characteristics of uniform overall brightness and brighter upper halves.