Diaspore is a popular color gemstone in the jewelry market. It is popular with consumers because of its unique Alexandrite effect (brownish green in the sunlight and purplish red in the incandescent lamp). The study on the Alexandrite effect origin of diaspore is of great significance to the cutting, the treatment and the value evaluation of gemstones. There are few studies on the Alexandrite effect origin of diaspore. Considering that the chemical composition and crystal structure of diaspore are similar to corundum, the theory on color origin of corundum is relatively mature. Therefore, to research the Alexandrite effect of diaspore, the corundum, which is very similar to the Alexandrite effect of diaspore, is selected for innovative comparative study from trace elements, UV-Vis spectrum and crystal structure. The trace elements in the samples were measured by laser ablation inductively coupled plasma mass spectrometer. According to the test results, the main chromogenic elements in diaspore are Fe, Cr, V, Ti and the main chromogenic elements in corundum are Mg, Ti, Fe, V, Cr. The types of trace elements in the two samples are similar, but the contents are different. Ultraviolet-visible and polarized ultraviolet-visible spectra were used to characterize the absorption characteristics of the samples in the visible light region. It was found that there were absorption peaks at 387, 398, 438 and 448 nm and absorption bands with centers at about 572 nm in the spectrum of diaspore. It was this absorption band that caused the Alexandrite effect. Correspondingly, there were absorption peaks at 377, 388 and 450 nm and an absorption band with the center at about 560 nm in the spectrum of diaspore. Their absorption characteristics in the visible light region are very similar. The difference is that the absorption band at 560 nm of corundum does not have obvious polarization, while the absorption band at 572 nm of diaspore has polarization. The absorption characteristics caused by charge transfer in crystals often have polarization. Through the comparative analysis of their crystal structures and the charge compensation theory in corundum, we speculate that the absorption peak at 398 nm in diaspore is caused by the d—d electron transition of Fe3+, the absorption peaks at 387, 438, 448 nm are caused by the Fe3+-Fe3+ pair, and the absorption band at 572 nm is caused by the Cr, V, Fe2+-Ti4+ pair. The alexandrite effect in diaspore is caused by Cr, V, Fe2+-Ti4+ pair. This study innovatively studied the Alexandrite effect origin of diaspore by comparing it with corundum, providing a new idea for studying similar problems in gemstones.