The most common type of garnet is pyrope-almandine, whose color varies from red to violet. In this study, 36 faceted gem-quality samples are used for electron microprobe, infrared spectrum, and UV-Visible spectrum test to find the coloration mechanism and spectroscopic characters in red-violet pyrope-almandine. The gradually increasing content of Mg2+ at the X position in the lattice is connected to the variation in the infrared spectrum. The wavenumber increases with the decrease of cationic radius, which makes the distance between C and D bands further. The color mechanism is mainly affected by Fe2+, and Mn2+. We discuss the change in colors with the assumption of a certain MnO content. When it is above or below the standard of 1 wt%, the absorption intensity of the UV-Visible spectrum is completely different in the purple zone, which determines the color to be red or violet. Therefore, the effect of Mn2+ and Fe2+ should be combined instead of being considered respectively. The results show that the MnO content can be quickly inferred by the light purple/fancy purple color. To avoid harming the value, this new insight makes it possible to quickly classify the gem quality in mining as well as in the market.