corundum, sapphire, Kenya.12, Turkana, asterism, inclusions.5, gems

Investigates the gemmological properties, inclusions, and causes of asterism in ten sapphire cabochons from northwestern Kenya, where commercially available rough began appearing on the local market in mid-1987. The material was reportedly collected from Quaternary surface sediments west of Lake Turkana, but its primary geological source remained unidentified. The samples ranged from blue through greenish-blue and greenish-yellow to dark brownish or nearly black, with transparency varying from moderately transparent to opaque. Eight displayed asterism, including six- and twelve-rayed effects, while two showed only concentrated silky patches. Four stones exhibited particularly sharp six-rayed stars with equally bright, narrow rays extending across the cabochons. Standard gemmological testing gave an approximate spot refractive index of 1.765, moderate pleochroism, and no reaction under long- or short-wave ultraviolet radiation or through the Chelsea filter. Most specimens showed iron-related absorption lines at 450, 460, and 470 nm; darker specimens instead showed a faint 470 nm line and a band between 450 and 460 nm. Microscopy, electron microprobe analysis, and X-ray diffraction distinguished cloudy areas, acicular features, silk, tubular cavities, transparent crystals, and feathers or healed fractures. The acicular systems responsible for asterism comprised microstriae chemically resembling the host corundum and needles attributed to rutile and hematite. Their crystallographic alignment and distribution governed the strength and completeness of the stars. Fine rutile needles contributed to silk, while hematite and other iron compounds were associated with darker colours. Additional inclusions comprised colourless tabular corundum crystals, brown prismatic crystals interpreted as rutile, and iron oxides or hydroxides filling channels and healed fractures. Angular blue colour zoning and inclusion arrangements were interpreted as records of successive growth episodes under changing physical and chemical conditions. Later fluid penetration introduced or redistributed iron compounds along fractures and cavities. Haloes around healed areas were tentatively linked to dehydration of iron hydroxides and formation of oxides during increasing temperature. The authors inferred high-temperature, high-pressure growth conditions and proposed that maximum crystallisation temperatures exceeded 500 °C. Although the finest stars closely resembled those of Linde synthetic star sapphires, straight angular growth zoning and characteristic iron absorption provided distinguishing evidence of natural origin. The study emphasises that an exceptionally regular star does not alone establish synthetic origin and that inclusion examination and spectroscopy are needed to characterise these Kenyan sapphires.