inclusions, garnet, gems

Gübelin examines the diagnostic value of inclusions in garnets, arguing that their internal mineral associations can provide a rapid means of identifying stones whose outward appearance resembles other gems. Such associations also preserve evidence of the circumstances of crystallization. The practical benefits are particularly apparent when instruments are unavailable or when numerous small stones require sorting, making individual measurements of specific gravity and optical properties laborious. Illustrated with 22 photomicrographs, the three installments describe characteristic features of demantoid, hessonite, pyrope, almandine, rhodolite, and spessartine. In demantoid, Gübelin reports fine brown or colorless fibers, identified in the article as byssolite, arranged in curved bundles, radiating clusters, or irregular tangles. These conspicuous inclusions help distinguish the green garnet from emerald and peridot and may be recognizable with a pocket loupe. Hessonite is characterized by numerous enclosed diopside crystals and streaks that produce its granular, internally oily appearance, providing a distinction from similarly colored zircon, tourmaline, citrine, and glass. Pyrope, almandine, and rhodolite are less readily separated from one another by inclusions, although their internal features assist in distinguishing them from other red and purple gemstones. Gübelin describes dark needles and rods, tentatively attributed to hornblende, arranged in three dimensions according to the host’s crystallography, contrasting this arrangement with the predominantly planar distribution of rutile silk in corundum. Other examples include thin birefringent tablets, sharply formed quartz crystals in Bohemian garnets, and circular arrangements of unidentified crystals possibly representing augite. Sri Lankan almandines contain abundant minute crystals, tubular liquid inclusions, and zircon crystals surrounded by radiation halos; specimens from near Trincomalee are described as densely packed with coarse hornblende inclusions. The concluding installment considers the concentric discoloration rings surrounding radioactive zircon inclusions. Gübelin suggests that differences in the range of alpha particles within different host minerals might eventually provide an additional diagnostic criterion, while presenting this as a preliminary research possibility requiring further investigation. Spessartine is distinguished by wavy, irregular feathers composed of minute liquid inclusions, occurring singly or in bundles and resembling fluid feathers in rubellite. Examination with a polarizing microscope, or a Diamondscope combined with a polariscope, also reveals the gratinglike extinction patterns attributed to strain-induced anomalous birefringence in garnet. Although acknowledging that inclusion study cannot identify every specimen, especially flawless material, Gübelin advocates familiarity with these internal features as an efficient way to reduce identification errors, using both microscopy and an ordinary jeweler’s loupe.