gems, corundum, inclusions.5, Sri Lanka.12, garnet, inclusions

Solid mineral inclusions in six corundum specimens and four almandine garnets collected by the author from gem gravels in the Ratnapura district of Ceylon (Sri Lanka) were investigated using optical examination and X-ray powder diffraction. Their collection establishes their geographical provenance, although the deposits are secondary. The pebbles were partly faceted to permit refractometer measurements, microscopic examination and access to inclusions intersecting the surface. Inclusions were photographed before material was removed, and specific gravities were determined hydrostatically. Powder diffraction photographs were obtained using iron radiation and a camera 114.6 mm in diameter, enabling identification from very small quantities of material where transparency, crystal size or orientation prevented reliable optical determination. In corundum, diffraction patterns identified magnesium spinel, pyrrhotite, apatite, phlogopite, rutile and corundum itself. Spinel occurred as irregular reddish-brown material and as dark octahedral crystals; a thin surface coating gave some transparent crystals a misleading metallic appearance. Pyrrhotite formed strongly magnetic, bronze-yellow grains, while phlogopite occurred as strongly dichroic brown plates. Apatite commonly formed colourless hexagonal prisms, and abundant apatite was suggested as the cause of unusually low specific gravities in two corundums. Rutile detected in material removed from one specimen was probably associated with its silk. The identification of corundum inclusions within corundum required repeated preparation: possible contamination by the host and an abrasive was resolved by isolating an entire inclusion, whose diffraction pattern confirmed corundum. A brownish-red powdery inclusion remained unidentified, although its diffraction data showed similarities to halloysite and serpentine-group minerals. In almandine garnet, apatite, rutile and muscovite were identified. Apatite displayed markedly variable appearances, including rounded, apparently isometric grains, elongated hexagonal prisms and a large inclusion resembling calcite with rhombohedral cleavage. These examples demonstrate the limitations of identification from morphology alone. Including additional specimens outside the detailed descriptions, apatite had been found in three corundums, nine garnets and one spinel; all gave identical powder patterns assigned to chlorine-bearing hydroxylapatites. Additional observations included calcite in two rubies and sphalerite in two garnets. Apatite and spinel are considered relatively common inclusions, but whether the reported mineral associations are diagnostic of Ceylon provenance remains unresolved and requires further investigation.