The impurity transition element chromium Cr3+ in ruby can be excited to high energy levels by absorbing visible light. When the excited Cr3+ returns to ground state through an intermediate energy level, the ruby emits red light as fluorescence. The fluorescence of ruby is actually caused by visible light, not by ultraviolet. The color of ruby is attributed to both the absorption and the fluorescence. The spectrum of a natural ruby under visible light is directly measured by a dual integrating sphere spectrometer. The measured spectrum includes both the spectral reflectance and the fluorescence caused by visible light. This study confirmed that the red fluorescence of ruby is indeed caused by visible light, not by ultraviolet radiation. In addition, average color of the ruby is calculated from the measured spectrum, and true color grade is also obtained by the artificial intelligent software of the spectrometer. (c) 2012 Wiley Periodicals, Inc. Col Res Appl, 38, 328-333, 2012