gems, spinel, Vietnam, Luc Yen

Gem spinel from northern Vietnam is described through market observations and gemmological examination of eleven faceted specimens weighing 0.36–4.30 ct. Spinel occurs with corundum in the Vietnamese deposits, paralleling associations familiar from Sri Lanka and Myanmar. According to information obtained by Kenneth Scarratt from a knowledgeable Vietnamese colleague, gem spinel was discovered in Vietnam in 1984, before the discovery of ruby. By the time of this report, substantial quantities in varied colours and sizes were available in local markets, including Luc Yen. Saverio Repetto of the Gemmological Institute of Vietnam supplied the stones examined, which ranged from orangy red to purple, orangy pink to purplish pink, and violet to blue. Their measured properties agreed with published data for spinel: refractive indices were 1.714–1.719, hydrostatically determined specific gravities were 3.59–3.63, and weak to moderate anomalous birefringence was observed between crossed polarisers. Ultraviolet fluorescence varied with body colour. Predominantly red and pink stones showed weak to moderate red fluorescence under long-wave radiation, whereas specimens with a stronger purple component reacted more weakly or remained inert. Short-wave reactions were similar in colour but less intense, and the blue stones were inert under both wavelengths. Examination with a desk-model spectroscope revealed the characteristic organ-pipe fluorescence spectrum in most stones containing a pink or red component; blue specimens displayed spectral features attributed to iron. Microscopy disclosed inclusions and growth features already recorded in spinels from other localities. On the basis of appearance, mineral inclusions were tentatively identified as hexagonal muscovite platelets, apatite crystals, zircon crystals surrounded by radiation-induced strain halos, small opaque black graphite crystals, and bundles of fine parallel or intersecting rutile silk. Other internal characteristics included secondary healing planes forming fingerprint inclusions, decorated intergrowths and ribbon-like stepped growth planes. The note documents a broad colour range in Vietnamese spinel and records conventional physical properties, colour-related fluorescence and spectroscopic responses, together with a varied inclusion assemblage whose mineral identifications remain provisional.