Garnet has many species because of its common isomorphism. In this study, a suite of 25 natural gem-quality garnets, including pyrope, almandine, spessartine, grossular, and andradite, were examined by standard gemological testing, LA-ICP-MS, FTIR, and Raman analysis. Internal stretching and bending vibrations of the SiO₄-tetrahedra of garnet exhibit correlate with the type of cations in garnet’s dodecahedral position (A site) and octahedral position (B site). FTIR and Raman spectra showed that with the increase of the radius of Mg²⁺, Fe²⁺, Mn²⁺, and Ca²⁺ in A site, or the unit cell volumes of pyrope, almandine, spessartine, and grossular, the spectral peaks of Si–O_str and Si–O_bend modes shift to low wavenumber. Because of the largest cations both in A site (Ca²⁺) and in B site (Fe³⁺), andradite exhibited the lowest wavenumber of Si–O_str and Si–O_bend modes of the five garnet species. Therefore, garnet has correlations between chemical composition and vibration spectroscopy, and Raman or IR spectroscopy can be used to precisely identify garnet species.

Hessonite, a special variety of grossularite, is well-known for the heat-wave effect, which is a characteristic swirled or roiled interior appearance within the crystal. Although the heat-wave effect has been observed for a long time, it has not been studied in depth. In this study, the gemological properties, mineral compositions, fabric characteristics, and grain sizes of hessonite samples were investigated using infrared spectroscopy, electron backscatter diffraction (EBSD), and energy-dispersive X-ray spectroscopy (EDS). Hessonite exhibits the heat-wave effect and is found to be polycrystal rather than single-crystal, composed of submillimeter-sized granules with random orientation and limited variations in Fe and Al contents. Abundant micropores exist among the granules, indicating imperfect contact among them. Due to these structural features, incident light is interrupted and undergoes changes in direction and speed as it passes through the hessonite granules, grain borders, and micropores. Light reflects off the granules’ surfaces and refracts within the granules, respectively, causing the incident light to swirl and roil within the hessonite and form the heat-wave effect. This study considers that the heat-wave effect is a special optical phenomenon not caused by impurity minerals or inclusions.