corundum, UK.12, Scotland.12, geology.11, gems

An occurrence of unusually large Scottish sapphires is described from a xenolith-bearing monchiquite dyke at Loch Roag, Isle of Lewis, in the Outer Hebrides. The dyke intrudes the Lewisian Complex and is exposed over approximately 24 m, with a width varying from 0.5 to 1.5 m. Its coarse central zone contains xenoliths of presumed mantle and deep crustal origin together with corroded megacrysts of biotite, augite, apatite, feldspar and scarce corundum. Sapphire crystals average 2–3 cm and reach 46 mm in diameter, substantially exceeding the sizes generally recorded from earlier Scottish occurrences. They commonly form distorted, striated, truncated hexagonal pyramids and bipyramids, with occasional tabular prismatic crystals. Smaller specimens tend to be fine blue, whereas larger crystals commonly contain patchy blue, green and occasional yellow areas. Fracturing is universal, often accompanied by secondary iron staining, and some crystals show basal parting. One specimen displays a hexagonal radial structure and sector zoning attributed to symmetrically distributed hematite exsolution lamellae. Detailed inclusion study was restricted because most crystals were retained as mineral specimens. A faceted 2.7 ct sapphire contained a feldspar grain, opaque breadcrumb-like inclusions and dark wispy veils associated with fractures; niobium-rich rutile is also reported in some specimens. Measured refractive indices are 1.762–1.767 for the extraordinary ray and 1.772–1.775 for the ordinary ray, with birefringence of 0.008–0.010 and specific gravity of 3.95–4.05. The sapphires show pronounced pleochroism, a broad diffuse absorption band centred near 450 nm and no fluorescence under long-wave or short-wave ultraviolet radiation. Geological comparisons suggest affinities with sapphire-bearing alkaline rocks in Southeast Asia and Australia, despite differences in age and emplacement. A proposed source is the disruption of peraluminous pegmatoids in the upper mantle or lower crust. The author considers commercial prospects limited by the small volume of exposed sapphire-bearing rock, the scarcity of crystals and the absence of demonstrated alluvial concentrations. Nevertheless, the discovery provides Scottish sapphire of cuttable size and raises the possibility that additional, perhaps wider, dykes remain concealed beneath peat.