corundum, ruby, Tanzania.12, East Africa.12, gems

Bank describes a ruby-bearing zoisite-amphibolite from Tanganyika (present-day Tanzania), a material that had recently attracted attention both mineralogically and as an ornamental stone. The rock consists principally of green zoisite and dark amphibole containing conspicuous red to purplish-red ruby crystals, producing the striking color contrast characteristic of the material. The constituent minerals and their physical and optical properties are examined, with particular attention to the ruby and zoisite, and the occurrence is considered in relation to its geological setting and mineral associations. The paper discusses the identification and gemological characteristics of the material and its suitability for cutting and polishing, documenting an unusual natural association in which ruby occurs embedded in a contrasting green zoisite-rich matrix.