gems, spinel, Burma, Vietnam, Tanzania, Tunduru, Madagascar, Mogok, An Phu, Lang Chap

This study investigates the characteristics and color variations of spinels from main deposits, including Liolava in Madagascar, Tunduru in Tanzania, Mogok in Myanmar, and three deposits in Vietnam (Minh Tien, Lang Chap, and An Phu). These spinel samples are categorized into four color varieties, i.e., red, orange, blue, and purple. All samples yielded the common properties of spinels. In addition, most red and orange spinels display strong red fluorescence under long-wave ultraviolet (LWUV) and short-wave ultraviolet (SWUV) lamps; however, some orange spinels exhibit non-fluorescence to red fluorescence under SWUV lamps. Blue spinel displays non-fluorescence under LWUV and SWUV lamps, whereas purple spinels exhibit non-fluorescence to strong red fluorescence under LWUV lamps. Ultraviolet-Visible-Near Infrared (UV-Vis-NIR) absorption spectra of red spinels display absorptions in violet and green to yellow regions, resulting in a red hue, while orange spinels exhibit broad absorption in violet to parts of the blue spectrum and weak absorption in the green to yellow spectrum, resulting in an orange hue. Regarding blue and purple spinels, they show strong absorption bands in the violet spectrum. However, various absorption bands in green to red regions affect the blue and purple hue. Mineral chemistries of all spinels show end-member proportions of spinel (MgAl₂O₄). However, red spinels reveal a higher proportion of magnesiochromite (MgCr₂O₄) than orange spinels. Blue spinels exhibit a higher proportion of hercynite (FeAl₂O₄) than purple spinels. Regarding their trace elements, Mn against Feₜₒₜₐₗ effectively distinguishes blue spinels from Liolava, Tunduru, Mogok, Minh Tien, and An Phu. Differentiation of purple spinels from Tunduru, Mogok, and An Phu, as well as red spinel origins from Mogok, Lang Chap, and An Phu, is achieved through Ti against Zn. Furthermore, V against Zn proves to be a discriminative factor for origin determination of orange spinels from Lang Chap and An Phu; however, further study is required to clarify their overlapping with Mogok spinels.