gems, spinel, spinel inclusions, chemistry, SEM, Vietnam, Sri Lanka, corundum, zoisite, allanite, paragonite, k-feldspar, plagioclase, florencite, biotite, pyrite, diopside, dolomite, calcite, titanite, rutile, zircon, graphite, pyrrhotite, pentlandite, violarite, clinochlore, phlogopite, magnetite, chondrodite

A study on the chemistry and gemmological features of spinels from Ratnapura in Sri Lanka and Luc Yen in Vietnam using scanning electron microscopy. The research identifies chromophore oxides responsible for coloration, noting that V³⁺ determines yellow and brown hues, Cr³⁺ drives red and pink tones, and Fe²⁺ with low Co²⁺ produces purple or violet shades. Mineral inclusions were cataloged, with Vietnamese spinels containing dolomite, calcite, titanite, rutile, zircon, graphite, apatite, pyrrhotite, pentlandite, violarite, clinochlore, phlogopite, magnesite and chondrodite, while Sri Lankan samples featured allanite, paragonite, zoisite, K-feldspar, plagioclase, florencite, biotite, pyrite and diopside. The authors conclude that vanadium oxide content above 0.4 wt.% is critical for orange and brown coloration in spinels, complementing existing models that primarily rely on chromium, iron and cobalt.