gems, beryl, emerald, Colombia, crystallography, morphology

The morphology of Colombian emerald, including growth and dissolution features of their faces, gives insight into growth and post-growth history of individual crystals. A collection of 15 isolated crystals and seven emeralds in matrix was studied by optical methods. The authors observed layered growth of prismatic, dipyramidal, and basal sectors that leads to crystals with prismatic to columnar or rarely pyramidal habit. In the latter case, prismatic growth sectors were not developed. The visual appearance of the crystal faces at the surface was influenced by etch patterns with the formation of cavities, pits, pointed hillocks, and stepped grooves, followed occasionally by layered overgrowth. Skeletal growth led to the formation of cavities in crystals, some of them resembling empty cups with planar bottoms. In these crystals, the basal faces showed indentations or deep cavities, surrounded by shells or rims of emerald, bound on both vertical surfaces by prism or dipyramidal faces. Emeralds in the form of slightly conical empty tubes also belong to the latter group. Possible relationships to trapiche emeralds and samples exhibiting the gota de aceite effect are discussed.