Abstract: Standard gemmological equipments, electron probe microanalyzer and fiber optic spectrometer were used to study a alexandrite effect garnet,which presents phenomenon as purple, blue and green under different light resources. The chemical formula of sample was calculated based on EPMA data and the subspecies name is determined as iron calcium-bearing manganic pyrope by Grs-Prp-(Alm+Sps) diagram plotting. The alexandrite effect is dominated by V, with contribution of Mn, Fe and Cr. The wide absorption band with 450 nm as the center is caused by Fe and Mn, while the absorption band at about 575 nm as center is the result of V and Cr. The intensity approach of the two wide absorption band in visible light is the direct cause of the garnet sample's colour change under incandescent and fluorescent light. Strong absorption of yellow green area makes the garnet sample display purple under incandescent light. Strong absorption of orange area makes the garnet sample display blue under weak fluorescent lamp, while wide and strong absorption of red area makes it show green under strong fluorescent light. The thin and low transparency area with 508 nm as the center is caused by absorption of rich V and spreads on mid-short wave area coupling with Mn and Fe, which plays an important role in the rare tri-colour change effect of the garnet sample.