Corundum, ruby, Nepal.12, inclusions.5, gems

Bank and colleagues describe a high-quality faceted ruby reportedly from Nepal whose unusual growth structures and inclusions initially raised doubts about its natural origin. The 1.288 ct stone, cut as an approximately equilateral octagon measuring 6.00 × 6.00 × 4.15 mm, combined excellent transparency with a desirable red colour, contrasting with the predominantly cabochon-quality Nepalese material previously available. Routine measurements gave refractive indices of 1.770 and 1.762, birefringence of 0.008, density of 3.98 g/cm³, and the normal chromium absorption spectrum, properties compatible with both natural and synthetic ruby. Ultraviolet–visible spectrophotometry showed similarities to previously studied Nepalese corundum but lacked significant iron–titanium charge-transfer absorption in the red region, accounting for the absence of a purplish sapphire component. Immersion microscopy revealed a dark red centre, opposing near-colourless marginal zones, and broad alternating red and colourless bands parallel to the basal pinacoid. Boundaries associated with prism and dipyramidal growth formed angles of approximately 90° and 85° with the basal zoning. Although superficially comparable to nearly rectangular zoning in Ramaura synthetic ruby, these structures involved different crystallographic forms. Spindle-like growth features within the red centre also resembled structures encountered in flux-grown synthetics, while fine dust-like striations recalled inclusions in Kashan synthetic ruby. These similarities made straightforward microscopic classification unreliable. Electron microprobe examination of surface-reaching inclusions at the girdle and near the culet identified phlogopite, a mica already known in association with Nepalese ruby and pink and violet sapphire. Further microscopy disclosed feathers comprising irregular cavities and negative crystals containing liquid, two-phase fillings, and probable three-phase solid–liquid–gas inclusions. Some solid components displayed interference colours under crossed polarizers. These feathers could resemble residual flux in synthetic ruby, but comparable features had previously been recorded in lower-quality Nepalese specimens. Extremely thin fluid films, sometimes containing both liquid and gas, provided additional evidence of natural formation and showed interference colours under suitable illumination. The authors conclude that the combined presence of phlogopite, multiphase inclusions, and ultrafine fluid films establishes natural origin and confirms the reported Nepalese provenance. The case demonstrates how exceptional natural growth structures can imitate familiar synthetic features, requiring crystallographic interpretation, inclusion identification, and multiple complementary observations rather than reliance on superficial resemblance.