gems, garnet, East Africa, Tanzania, Umba River, Umba Valley, pyrope-almandine, pyralspite, umbalite

A series of 204 variously coloured garnet pebbles was acquired by J. Saul during 1974 from the same alluvial area along the Umba River as the blue colour-change garnet described by Jobbins et al. 1975. The garnets were shown to us at the Institute and it was suggested that they would form an ideal subject for a detailed study of a large sample from a known and relatively restricted provenance. A small polished flat was placed on each pebble to facilitate optical determinations. After cleaning in an ultrasonic cleaner the pebbles were sorted into thirteen groups on a colour basis by one author (P.M.S.) and a second author (E.A.J.) checked and agreed with this grouping, which greatly assisted the subsequent handling. At this stage it was noted that the stones in groups 12 and 13 showed marked colour changes (greys to pinks) from daylight to tungsten illumination, that those in group 11 appeared khaki by transmitted daylight but pale orange-brown by reflected daylight, and that there was more variation within the suite than had been apparent to any of us at first inspection.