The invention concerns a method for qualifying a diamond on the basis of a measured light transmission through the diamond, whereby the diamond is radiated by a light source which emits light having a wavelength in a range of 225 nm to 300 nm, whereby the transmission of said light through the diamond is compared to a reference value which corresponds to the transmission of said light through a reference diamond, which is a cut colourless or near colourless diamond with a concentration of A centers between 7 ppm and 22 ppm, and whereby the diamond is classified as natural and not colour-treated if the transmission through the diamond is smaller than or equal to the reference value.