gems, Ceylon, Sri Lanka, chrysoberyl, twinning, cyclic twinning, chrysoberyl flowers

Goldschmidt and Preiswerk present a detailed crystallographic study of an exceptionally well-formed chrysoberyl twin brought from Ceylon by C. Schmidt. The wine-yellow crystal measures 11 × 6 × 5 mm. Goniometric measurements revealed several previously unrecorded crystal forms and permitted detailed analysis of the twin relationship. The authors identify the normal chrysoberyl twin law with (031) as the twin plane and use gnomonic projection to demonstrate the geometrical relationship between the two individuals. They calculate revised crystallographic elements from their measurements, compare them with those of Haidinger and contemporary measurements by G. Melczer, and provide angular tables for two crystallographic orientations. The observed forms are further analyzed in terms of their development within three principal zones, leading to a structural interpretation in which an underlying hexagonal arrangement produces the observed orthorhombic symmetry through differing preferred directions of the primary nodes.